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Title: Volcanic Islands

Author: Charles Darwin

Release date: February 1, 2002 [eBook #3054]
                Most recently updated: March 29, 2021

Language: English

Credits: Sue Asscher


*** START OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS ***




Geological Observations on the Volcanic Islands

by Charles Darwin


Contents

 EDITORIAL NOTE
 DETAILED TABLE OF CONTENTS
 CRITICAL INTRODUCTION
 CHAPTER I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO
 CHAPTER II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC
 CHAPTER III.—ASCENSION
 CHAPTER IV.—ST. HELENA
 CHAPTER V.—GALAPAGOS ARCHIPELAGO
 CHAPTER VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES
 CHAPTER VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE
 INDEX TO VOLCANIC ISLANDS




EDITORIAL NOTE


Although in some respects more technical in their subjects and style
than Darwin’s “Journal,” the books here reprinted will never lose their
value and interest for the originality of the observations they
contain. Many parts of them are admirably adapted for giving an insight
into problems regarding the structure and changes of the earth’s
surface, and in fact they form a charming introduction to physical
geology and physiography in their application to special domains. The
books themselves cannot be obtained for many times the price of the
present volume, and both the general reader, who desires to know more
of Darwin’s work, and the student of geology, who naturally wishes to
know how a master mind reasoned on most important geological subjects,
will be glad of the opportunity of possessing them in a convenient and
cheap form.

The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.

G.T.B.




VOLCANIC ISLANDS.




DETAILED TABLE OF CONTENTS


I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.

Rocks of the lowest series.—A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent.—Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriae.—Ancient and
obliterated orifices of eruption of small size.—Difficulty of tracing
over a bare plain recent streams of lava.—Inland hills of more ancient
volcanic rock.—Decomposed olivine in large masses.—Feldspathic rocks
beneath the upper crystalline basaltic strata.—Uniform structure and
form of the more ancient volcanic hills.—Form of the valleys near the
coast.—Conglomerate now forming on the sea beach.

II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.

FERNANDO NORONHA.—Precipitous hill of phonolite.

TERCEIRA.—Trachytic rocks: their singular decomposition by steam of
high temperature.

TAHITI.—Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype.

MAURITIUS.—Proofs of its recent elevation.—Structure of its more
ancient mountains; similarity with St. Jago.

ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular mineralogical
composition.

III.—ASCENSION.

Basaltic lavas.—Numerous craters truncated on the same side.—Singular
structure of volcanic bombs.—Aeriform explosions.—Ejected granite
fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
formation.—Concretions in pumiceous tuff.—Calcareous deposits and
frondescent incrustations on the coast.—Remarkable laminated beds,
alternating with, and passing into obsidian.—Origin of obsidian.—
Lamination of volcanic rocks.

IV.—ST. HELENA.

Lavas of the feldspathic, basaltic, and submarine series.—Section of
Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous Bays.—
Basaltic ring.—Central crateriform ridge, with an internal ledge and a
parapet.—Cones of phonolite.—Superficial beds of calcareous sandstone.—
Extinct land-shells.—Beds of detritus.—Elevation of the land.—
Denudation.—Craters of elevation.

V.—GALAPAGOS ARCHIPELAGO.

Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
basaltic craters, with hollows at their bases.—Albemarle Island; fluid
lavas, their composition.—Craters of tuff; inclination of their
exterior diverging strata, and structure of their interior converging
strata.—James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected
fragments.—Concluding remarks on the craters of tuff, and on the
breached condition of their southern sides.— Mineralogical composition
of the rocks of the archipelago.—Elevation of the land.—Direction of
the fissures of eruption.

VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.

The sinking of crystals in fluid lava.—Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation.— Obsidian.—Apparent non-separation of the elements of
plutonic rocks.— Origin of trap-dikes in the plutonic
series.—Distribution of volcanic islands; their prevalence in the great
oceans.—They are generally arranged in lines.—The central volcanoes of
Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
volcanic islands, and their elevation in mass.—Eruptions on parallel
lines of fissure within the same geological period.

VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.

New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
Land.—Palaeozoic formation.—Newer formation with volcanic rocks.—
Travertin with leaves of extinct plants.—Elevation of the land.—New
Zealand.—King George’s Sound.—Superficial ferruginous beds.—Superficial
calcareous deposits, with casts of branches; its origin from drifted
particles of shells and corals.—Their extent.—Cape of Good Hope.—
Junction of the granite and clay-slate.—Sandstone formation.

INDEX.




GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.




CRITICAL INTRODUCTION.


The preparation of the series of works published under the general
title “Geology of the Voyage of the ‘Beagle’” occupied a great part of
Darwin’s time during the ten years that followed his return to England.
The second volume of the series, entitled “Geological Observations on
Volcanic Islands, with Brief Notices on the Geology of Australia and
the Cape of Good Hope,” made its appearance in 1844. The materials for
this volume were collected in part during the outward voyage, when the
“Beagle” called at St. Jago in the Cape de Verde Islands, and St.
Paul’s Rocks, and at Fernando Noronha, but mainly during the homeward
cruise; then it was that the Galapagos Islands were surveyed, the Low
Archipelago passed through, and Tahiti visited; after making calls at
the Bay of Islands, in New Zealand, and also at Sydney, Hobart Town and
King George’s Sound in Australia, the “Beagle” sailed across the Indian
Ocean to the little group of the Keeling or Cocos Islands, which Darwin
has rendered famous by his observations, and thence to Mauritius;
calling at the Cape of Good Hope on her way, the ship then proceeded
successively to St. Helena and Ascension, and revisited the Cape de
Verde Islands before finally reaching England.

Although Darwin was thus able to gratify his curiosity by visits to a
great number of very interesting volcanic districts, the voyage opened
for him with a bitter disappointment. He had been reading Humboldt’s
“Personal Narrative” during his last year’s residence in Cambridge, and
had copied out from it long passages about Teneriffe. He was actually
making inquiries as to the best means of visiting that island, when the
offer was made to him to accompany Captain Fitzroy in the “Beagle. “
His friend Henslow too, on parting with him, had given him the advice
to procure and read the recently published first volume of the
“Principles of Geology,” though he warned him against accepting the
views advocated by its author. During the time the “Beagle” was beating
backwards and forwards when the voyage commenced, Darwin, although
hardly ever able to leave his berth, was employing all the
opportunities which the terrible sea-sickness left him, in studying
Humboldt and Lyell. We may therefore form an idea of his feelings when,
on the ship reaching Santa Cruz, and the Peak of Teneriffe making its
appearance among the clouds, they were suddenly informed that an
outbreak of cholera would prevent any landing!

Ample compensation for this disappointment was found, however, when the
ship reached Porta Praya in St. Jago, the largest of the Cape de Verde
Islands. Here he spent three most delightful weeks, and really
commenced his work as a geologist and naturalist. Writing to his father
he says, “Geologising in a volcanic country is most delightful; besides
the interest attached to itself, it leads you into most beautiful and
retired spots. Nobody but a person fond of Natural History can imagine
the pleasure of strolling under cocoa-nuts in a thicket of bananas and
coffee-plants, and an endless number of wild flowers. And this island,
that has given me so much instruction and delight, is reckoned the most
uninteresting place that we perhaps shall touch at during our voyage.
It certainly is generally very barren, but the valleys are more
exquisitely beautiful, from the very contrast. It is utterly useless to
say anything about the scenery; it would be as profitable to explain to
a blind man colours, as to a person who has not been out of Europe, the
total dissimilarity of a tropical view. Whenever I enjoy anything, I
always look forward to writing it down, either in my log-book (which
increases in bulk), or in a letter; so you must excuse raptures, and
those raptures badly expressed. I find my collections are increasing
wonderfully, and from Rio I think I shall be obliged to send a cargo
home.”

The indelible impression made on Darwin’s mind by this first visit to a
volcanic island, is borne witness to by a remarkable passage in the
“Autobiography” written by him in 1876. “The geology of St. Jago is
very striking, yet simple; a stream of lava formerly flowed over the
bed of the sea, formed of triturated recent shells and corals, which it
has baked into a hard white rock. Since then the whole island has been
upheaved. But the line of white rock revealed to me a new and important
fact, namely that there had been afterwards subsidence round the
craters which had since been in action, and had poured forth lava. It
then first dawned on me that I might perhaps write a book on the
geology of the various countries visited, and this made me thrill with
delight. That was a memorable hour to me, and how distinctly I can call
to mind the low cliff of lava beneath which I rested, with the sun
glaring hot, a few strange desert plants growing near and with living
corals in the tidal pools at my feet.”

Only five years before, when listening to poor Professor Jameson’s
lectures on the effete Wernerianism, which at that time did duty for
geological teaching, Darwin had found them “incredibly dull,” and he
declared that “the sole effect they produced on me was a determination
never so long as I lived to read a book on Geology, or in any way to
study the science.”

What a contrast we find in the expressions which he makes use of in
referring to Geological Science, in his letters written home from the
“Beagle!” After alluding to the delight of collecting and studying
marine animals, he exclaims, “But Geology carries the day!” Writing to
Henslow he says, “I am quite charmed with Geology, but, like the wise
animal between two bundles of hay, I do not know which to like best;
the old crystalline group of rocks, or the softer and more
fossiliferous beds.” And just as the long voyage is about to come to a
close he again writes, “I find in Geology a never-failing interest; as
it has been remarked, it creates the same grand ideas respecting this
world which Astronomy does for the Universe.” In this passage Darwin
doubtless refers to a remark of Sir John Herschel’s in his admirable
“Preliminary Discourse on the Study of Natural Philosophy,”—a book
which exercised a most remarkable and beneficial influence on the mind
of the young naturalist.

If there cannot be any doubt as to the strong predilection in Darwin’s
mind for geological studies, both during and after the memorable
voyage, there is equally little difficulty in perceiving the school of
geological thought which, in spite of the warnings of Sedgwick and
Henslow, had obtained complete ascendancy over his mind. He writes in
1876: “The very first place which I examined, namely St. Jago in the
Cape de Verde Islands, showed me clearly the wonderful superiority of
Lyell’s manner of treating Geology, compared with that of any other
author, whose works I had with me, or ever afterwards read.” And again,
“The science of Geology is enormously indebted to Lyell—more so, as I
believe, than to any other man who ever lived...I am proud to remember
that the first place, namely, St. Jago, in the Cape de Verde
Archipelago, in which I geologised, convinced me of the infinite
superiority of Lyell’s views over those advocated in any other work
known to me.”

The passages I have cited will serve to show the spirit in which Darwin
entered upon his geological studies, and the perusal of the following
pages will furnish abundant proofs of the enthusiasm, acumen, and
caution with which his researches were pursued.

Large collections of rocks and minerals were made by Darwin during his
researches, and sent home to Cambridge, to be kept under the care of
his faithful friend Henslow. After visiting his relations and friends,
Darwin’s first care on his return to England was to unpack and examine
these collections. He accordingly, at the end of 1836, took lodgings
for three months in Fitzwilliam Street, Cambridge, so as to be near
Henslow; and in studying and determining his geological specimens
received much valuable aid from the eminent crystallographer and
mineralogist, Professor William Hallows Miller.

The actual writing of the volume upon volcanic islands was not
commenced till 1843, when Darwin had settled in the spot which became
his home for the rest of his life—the famous house at Down, in Kent.
Writing to his friend Mr. Fox, on March 28th, 1843, he says, “I am very
slowly progressing with a volume, or rather pamphlet, on the volcanic
islands which we visited: I manage only a couple of hours per day, and
that not very regularly. It is uphill work writing books, which cost
money in publishing, and which are not read even by geologists.”

The work occupied Darwin during the whole of the year 1843, and was
issued in the spring of the following year, the actual time engaged in
preparing it being recorded in his diary as “from the summer of 1842 to
January 1844;” but the author does not appear to have been by any means
satisfied with the result when the book was finished. He wrote to
Lyell, “You have pleased me much by saying that you intend looking
through my ‘Volcanic Islands;’ it cost me eighteen months!!! and I have
heard of very few who have read it. Now I shall feel, whatever little
(and little it is) there is confirmatory of old work, or new, will work
its effect and not be lost.” To Sir Joseph Hooker he wrote, “I have
just finished a little volume on the volcanic islands which we visited.
I do not know how far you care for dry simple geology, but I hope you
will let me send you a copy.”

Every geologist knows how full of interest and suggestiveness is this
book of Darwin’s on volcanic islands. Probably the scant satisfaction
which its author seemed to find in it may be traced to the effect of a
contrast which he felt between the memory of glowing delights he had
experienced when, hammer in hand, he roamed over new and interesting
scenes, and the slow, laborious, and less congenial task of re-writing
and arranging his notes in book-form.

In 1874, in writing an account of the ancient volcanoes of the
Hebrides, I had frequent occasion to quote Mr. Darwin’s observations on
the Atlantic volcanoes, in illustration of the phenomena exhibited by
the relics of still older volcanoes in our own islands. Darwin, in
writing to his old friend Sir Charles Lyell upon the subject, says, “I
was not a little pleased to see my volcanic book quoted, for I thought
it was completely dead and forgotten.”

Two years later the original publishers of this book and of that on
South America proposed to re-issue them. Darwin at first hesitated, for
he seemed to think there could be little of abiding interest in them;
he consulted me upon the subject in one of the conversations which I
used to have with him at that time, and I strongly urged upon him the
reprint of the works. I was much gratified when he gave way upon the
point, and consented to their appearing just as originally issued. In
his preface he says, “Owing to the great progress which Geology has
made in recent times, my views on some few points may be somewhat
antiquated, but I have thought it best to leave them as they originally
appeared.”

It may be interesting to indicate, as briefly as possible, the chief
geological problem upon which the publication of Darwin’s “Volcanic
Islands” threw new and important light. The merit of the work consisted
in supplying interesting observations, which in some cases have proved
of crucial value in exploding prevalent fallacies; in calling attention
to phenomena and considerations that had been quite overlooked by
geologists, but have since exercised an important influence in moulding
geological speculation; and lastly in showing the importance which
attaches to small and seemingly insignificant causes, some of which
afford a key to the explanation of very curious geological problems.

Visiting as he did the districts in which Von Buch and others had found
what they thought to be evidence of the truth of “Elevation-craters,”
Darwin was able to show that the facts were capable of a totally
different interpretation. The views originally put forward by the old
German geologist and traveller, and almost universally accepted by his
countrymen, had met with much support from Elie de Beaumont and
Dufrenoy, the leaders of geological thought in France. They were,
however, stoutly opposed by Scrope and Lyell in this country, and by
Constant Prevost and Virlet on the other side of the channel. Darwin,
in the work before us, shows how little ground there is for the
assumption that the great ring-craters of the Atlantic islands have
originated in gigantic blisters of the earth’s surface which, opening
at the top, have given origin to the craters. Admitting the influence
of the injection of lava into the structure of the volcanic cones, in
increasing their bulk and elevation, he shows that, in the main, the
volcanoes are built up by repeated ejections causing an accumulation of
materials around the vent.

While, however, agreeing on the whole with Scrope and Lyell, as to the
explosive origin of ordinary volcanic craters, Darwin clearly saw that,
in some cases, great craters might be formed or enlarged, by the
subsidence of the floors after eruptions. The importance of this
agency, to which too little attention has been directed by geologists,
has recently been shown by Professor Dana, in his admirable work on
Kilauea and the other great volcanoes of the Hawaiian Archipelago.

The effects of subsidence at a volcanic centre in producing a downward
dip of the strata around it, was first pointed out by Darwin, as the
result of his earliest work in the Cape de Verde Islands. Striking
illustrations of the same principle have since been pointed out by M.
Robert and others in Iceland, by Mr. Heaphy in New Zealand, and by
myself in the Western Isles of Scotland.

Darwin again and again called attention to the evidence that volcanic
vents exhibit relations to one another which can only be explained by
assuming the existence of lines of fissure in the earth’s crust, along
which the lavas have made their way to the surface. But he, at the same
time, clearly saw that there was no evidence of the occurrence of great
deluges of lava along such fissures; he showed how the most remarkable
plateaux, composed of successive lava sheets, might be built up by
repeated and moderate ejections from numerous isolated vents; and he
expressly insists upon the rapidity with which the cinder-cones around
the orifices of ejection and the evidences of successive outflows of
lava would be obliterated by denudation.

One of the most striking parts of the book is that in which he deals
with the effects of denudation in producing “basal wrecks” or worn down
stumps of volcanoes. He was enabled to examine a series of cases in
which could be traced every gradation, from perfect volcanic cones down
to the solidified plugs which had consolidated in the vents from which
ejections had taken place. Darwin’s observations on these points have
been of the greatest value and assistance to all who have essayed to
study the effects of volcanic action during earlier periods of the
earth’s history. Like Lyell, he was firmly persuaded of the continuity
of geological history, and ever delighted in finding indications, in
the present order of nature, that the phenomena of the past could be
accounted for by means of causes which are still in operation. Lyell’s
last work in the field was carried on about his home in Forfarshire,
and only a few months before his death he wrote to Darwin: “All the
work which I have done has confirmed me in the belief that the only
difference between Palaeozoic and recent volcanic rocks is no more than
we must allow for, by the enormous time to which the products of the
oldest volcanoes have been subjected to chemical changes.”

Darwin was greatly impressed, as the result of his studies of volcanic
phenomena, followed by an examination of the great granite-masses of
the Andes, with the relations between the so-called Plutonic rocks and
those of undoubtedly volcanic origin. It was indeed a fortunate
circumstance, that after studying some excellent examples of recent
volcanic rocks, he proceeded to examine in South America many fine
illustrations of the older igneous rock-masses, and especially of the
most highly crystalline types of the same, and then on his way home had
opportunities of reviving the impression made upon him by the fresh and
unaltered volcanic rocks. Some of the general considerations suggested
by these observations were discussed in a paper read by him before the
Geological Society, on March 7th, 1838, under the title “On the
Connection of Certain Volcanic Phenomena, and On the Formation of
Mountain-chains, and the Effect of Continental Elevations.” The exact
bearing of these two classes of facts upon one another are more fully
discussed in his book on South American geology.

The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in
which upward movements were taking place, and he was naturally led to
contrast them with the areas in which, as he showed, the occurrence of
atolls, encircling reefs, and barrier-reefs afford indication of
subsidence. In this way he was able to map out the oceanic areas in
different zones, along which opposite kinds of movement were taking
place. His conclusions on this subject were full of novelty and
suggestiveness.

Very clearly did Darwin recognise the importance of the fact that most
of the oceanic islands appear to be of volcanic origin, though he was
careful to point out the remarkable exceptions which somewhat
invalidate the generalisation. In his “Origin of Species” he has
elaborated the idea and suggested the theory of the permanence of
ocean-basins, a suggestion which has been adopted and pushed farther by
subsequent authors, than we think its originator would have approved.
His caution and fairness of mind on this and similar speculative
questions was well-known to all who were in the habit of discussing
them with him.

Some years before the voyage of the “Beagle,” Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the
foliated crystalline schists. It does not appear that Darwin was
acquainted with this remarkable memoir, but quite independently he
called attention to the same phenomena when he came to study some very
similar rocks which occur in the island of Ascension. Coming fresh from
the study of the great masses of crystalline schist in the South
American continent, he was struck by the circumstance that in the
undoubtedly igneous rocks of Ascension we find a similar separation of
the constituent minerals along parallel “folia.” These observations led
Darwin to the same conclusion as that arrived at some time before by
Scrope—namely that when crystallisation takes place in rock masses
under the influence of great deforming stresses, a separation and
parallel arrangement of the constituent minerals will result. This is a
process which is now fully recognised as having been a potent factor in
the production of the metamorphic rock, and has been called by more
recent writers “dynamo-metamorphism.”

In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was in
making out the true facts with regard to the rocks he studied by the
simple aid of a penknife and pocket-lens, supplemented by a few
chemical tests and the constant use of the blowpipe. Since his day, the
method of study of rocks by thin sections under the microscope has been
devised, and has become a most efficient aid in all petrographical
inquiries. During the voyage of H.M.S. “Challenger,” many of the
islands studied by Darwin have been revisited and their rocks
collected. The results of their study by one of the greatest masters of
the science of micropetrography—Professor Renard of Brussels—have been
recently published in one of the volumes of “Reports on the
‘Challenger’ Expedition.” While much that is new and valuable has been
contributed to geological science by these more recent investigations,
and many changes have been made in nomenclature and other points of
detail, it is interesting to find that all the chief facts described by
Darwin and his friend Professor Miller have stood the test of time and
further study, and remain as a monument of the acumen and accuracy in
minute observation of these pioneers in geological research.

JOHN W. JUDD.




CHAPTER I.
ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.


Rocks of the lowest series. A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent. Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriae. Ancient and
obliterated orifices of eruption of small size. Difficulty of tracing
over a bare plain recent streams of lava. Inland hills of more ancient
volcanic rock. Decomposed olivine in large masses. Feldspathic rocks
beneath the upper crystalline basaltic strata. Uniform structure and
form of the more ancient volcanic hills. Form of the valleys near the
coast. Conglomerate now forming on the sea beach.

(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.)

The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty
miles in length by about twelve in breadth. My observations, made
during two visits, were confined to the southern portion within the
distance of a few leagues from Porto Praya. The country, viewed from
the sea, presents a varied outline: smooth conical hills of a reddish
colour (like Red Hill in Figure 1 (Map 1). (The outline of the coast,
the position of the villages, streamlets, and of most of the hills in
this woodcut, are copied from the chart made on board H.M.S. “Leven.”
The square-topped hills (A, B, C, etc.) are put in merely by eye, to
illustrate my description.)), and others less regular, flat-topped, and
of a blackish colour (like A, B, C,) rise from successive, step-formed
plains of lava. At a distance, a chain of mountains, many thousand feet
in height, traverses the interior of the island. There is no active
volcano in St. Jago, and only one in the group, namely at Fogo. The
island since being inhabited has not suffered from destructive
earthquakes.

The lowest rocks exposed on the coast near Porto Praya, are highly
crystalline and compact; they appear to be of ancient, submarine,
volcanic origin; they are unconformably covered by a thin, irregular,
calcareous deposit, abounding with shells of a late tertiary period;
and this again is capped by a wide sheet of basaltic lava, which has
flowed in successive streams from the interior of the island, between
the square-topped hills marked A, B, C, etc. Still more recent streams
of lava have been erupted from the scattered cones, such as Red and
Signal Post Hills. The upper strata of the square-topped hills are
intimately related in mineralogical composition, and in other respects,
with the lowest series of the coast- rocks, with which they seem to be
continuous.

MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES.

These rocks possess an extremely varying character; they consist of
black, brown, and grey, compact, basaltic bases, with numerous crystals
of augite, hornblende, olivine, mica, and sometimes glassy feldspar. A
common variety is almost entirely composed of crystals of augite with
olivine. Mica, it is known, seldom occurs where augite abounds; nor
probably does the present case offer a real exception, for the mica (at
least in my best characterised specimen, in which one nodule of this
mineral is nearly half an inch in length) is as perfectly rounded as a
pebble in a conglomerate, and evidently has not been crystallised in
the base, in which it is now enclosed, but has proceeded from the
fusion of some pre-existing rock. These compact lavas alternate with
tuffs, amygdaloids, and wacke, and in some places with coarse
conglomerate. Some of the argillaceous wackes are of a dark green
colour, others, pale yellowish-green, and others nearly white; I was
surprised to find that some of the latter varieties, even where
whitest, fused into a jet black enamel, whilst some of the green
varieties afforded only a pale gray bead. Numerous dikes, consisting
chiefly of highly compact augitic rocks, and of gray amygdaloidal
varieties, intersect the strata, which have in several places been
dislocated with considerable violence, and thrown into highly inclined
positions. One line of disturbance crosses the northern end of Quail
Island (an islet in the Bay of Porto Praya), and can be followed to the
mainland. These disturbances took place before the deposition of the
recent sedimentary bed; and the surface, also, had previously been
denuded to a great extent, as is shown by many truncated dikes.

DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC
ROCKS.

This stratum is very conspicuous from its white colour, and from the
extreme regularity with which it ranges in a horizontal line for some
miles along the coast. Its average height above the sea, measured from
the upper line of junction with the superincumbent basaltic lava, is
about sixty feet; and its thickness, although varying much from the
inequalities of the underlying formation, may be estimated at about
twenty feet. It consists of quite white calcareous matter, partly
composed of organic debris, and partly of a substance which may be
aptly compared in appearance with mortar. Fragments of rock and pebbles
are scattered throughout this bed, often forming, especially in the
lower part, a conglomerate. Many of the fragments of rock are
whitewashed with a thin coating of calcareous matter. At Quail Island,
the calcareous deposit is replaced in its lowest part by a soft, brown,
earthy tuff, full of Turritellae; this is covered by a bed of pebbles,
passing into sandstone, and mixed with fragments of echini, claws of
crabs, and shells; the oyster-shells still adhering to the rock on
which they grew. Numerous white balls appearing like pisolitic
concretions, from the size of a walnut to that of an apple, are
embedded in this deposit; they usually have a small pebble in their
centres. Although so like concretions, a close examination convinced me
that they were Nulliporae, retaining their proper forms, but with their
surfaces slightly abraded: these bodies (plants as they are now
generally considered to be) exhibit under a microscope of ordinary
power, no traces of organisation in their internal structure. Mr.
George R. Sowerby has been so good as to examine the shells which I
collected: there are fourteen species in a sufficiently perfect
condition for their characters to be made out with some degree of
certainty, and four which can be referred only to their genera. Of the
fourteen shells, of which a list is given in the Appendix, eleven are
recent species; one, though undescribed, is perhaps identical with a
species which I found living in the harbour of Porto Praya; the two
remaining species are unknown, and have been described by Mr. Sowerby.
Until the shells of this Archipelago and of the neighbouring coasts are
better known, it would be rash to assert that even these two latter
shells are extinct. The number of species which certainly belong to
existing kinds, although few in number, are sufficient to show that the
deposit belongs to a late tertiary period. From its mineralogical
character, from the number and size of the embedded fragments, and from
the abundance of Patellae, and other littoral shells, it is evident
that the whole was accumulated in a shallow sea, near an ancient
coast-line.

EFFECTS PRODUCED BY THE FLOWING OF THE SUPERINCUMBENT BASALTIC LAVA
OVER THE CALCAREOUS DEPOSIT.

These effects are very curious. The calcareous matter is altered to the
depth of about a foot beneath the line of junction; and a most perfect
gradation can be traced, from loosely aggregated, small, particles of
shells, corallines, and Nulliporae, into a rock, in which not a trace
of mechanical origin can be discovered, even with a microscope. Where
the metamorphic change has been greatest, two varieties occur. The
first is a hard, compact, white, fine-grained rock, striped with a few
parallel lines of black volcanic particles, and resembling a sandstone,
but which, upon close examination, is seen to be crystallised
throughout, with the cleavages so perfect that they can be readily
measured by the reflecting goniometer. In specimens, where the change
has been less complete, when moistened and examined under a strong
lens, the most interesting gradation can be traced, some of the rounded
particles retaining their proper forms, and others insensibly melting
into the granulo-crystalline paste. The weathered surface of this
stone, as is so frequently the case with ordinary limestones, assumes a
brick-red colour.

The second metamorphosed variety is likewise a hard rock, but without
any crystalline structure. It consists of a white, opaque, compact,
calcareous stone, thickly mottled with rounded, though regular, spots
of a soft, earthy, ochraceous substance. This earthy matter is of a
pale yellowish- brown colour, and appears to be a mixture of carbonate
of lime with iron; it effervesces with acids, is infusible, but
blackens under the blowpipe, and becomes magnetic. The rounded form of
the minute patches of earthy substance, and the steps in the progress
of their perfect formation, which can be followed in a suit of
specimens, clearly show that they are due either to some power of
aggregation in the earthy particles amongst themselves, or more
probably to a strong attraction between the atoms of the carbonate of
line, and consequently to the segregation of the earthy extraneous
matter. I was much interested by this fact, because I have often seen
quartz rocks (for instance, in the Falkland Islands, and in the lower
Silurian strata of the Stiper-stones in Shropshire), mottled in a
precisely analogous manner, with little spots of a white, earthy
substance (earthy feldspar?); and these rocks, there was good reason to
suppose, had undergone the action of heat,—a view which thus receives
confirmation. This spotted structure may possibly afford some
indication in distinguishing those formations of quartz, which owe
their present structure to igneous action, from those produced by the
agency of water alone; a source of doubt, which I should think from my
own experience, that most geologists, when examining arenaceo-quartzose
districts must have experienced.

The lowest and most scoriaceous part of the lava, in rolling over the
sedimentary deposit at the bottom of the sea, has caught up large
quantities of calcareous matter, which now forms a snow-white, highly
crystalline basis to a breccia, including small pieces of black, glossy
scoriae. A little above this, where the lime is less abundant, and the
lava more compact, numerous little balls, composed of spicula of
calcareous spar, radiating from common centres, occupy the interstices.
In one part of Quail Island, the lime has thus been crystallised by the
heat of the superincumbent lava, where it is only thirteen feet in
thickness; nor had the lava been originally thicker, and since reduced
by degradation, as could be told from the degree of cellularity of its
surface. I have already observed that the sea must have been shallow in
which the calcareous deposit was accumulated. In this case, therefore,
the carbonic acid gas has been retained under a pressure, insignificant
compared with that (a column of water, 1,708 feet in height) originally
supposed by Sir James Hall to be requisite for this end: but since his
experiments, it has been discovered that pressure has less to do with
the retention of carbonic acid gas, than the nature of the circumjacent
atmosphere; and hence, as is stated to be the case by Mr. Faraday,
masses of limestone are sometimes fused and crystallised even in common
limekilns. (I am much indebted to Mr. E.W. Brayley in having given me
the following references to papers on this subject: Faraday in the
“Edinburgh New Philosophical Journal” volume 15 page 398; Gay-Lussac in
“Annales de Chem. et Phys.” tome 63 page 219 translated in the “London
and Edinburgh Philosophical Magazine” volume 10 page 496.) Carbonate of
lime can be heated to almost any degree, according to Faraday, in an
atmosphere of carbonic acid gas, without being decomposed; and
Gay-Lussac found that fragments of limestone, placed in a tube and
heated to a degree, not sufficient by itself to cause their
decomposition, yet immediately evolved their carbonic acid, when a
stream of common air or steam was passed over them: Gay-Lussac
attributes this to the mechanical displacement of the nascent carbonic
acid gas. The calcareous matter beneath the lava, and especially that
forming the crystalline spicula between the interstices of the scoriae,
although heated in an atmosphere probably composed chiefly of steam,
could not have been subjected to the effects of a passing stream; and
hence it is, perhaps, that they have retained their carbonic acid,
under a small amount of pressure.

The fragments of scoriae, embedded in the crystalline calcareous basis,
are of a jet black colour, with a glossy fracture like pitchstone.
Their surfaces, however, are coated with a layer of a reddish-orange,
translucent substance, which can easily be scratched with a knife;
hence they appear as if overlaid by a thin layer of rosin. Some of the
smaller fragments are partially changed throughout into this substance:
a change which appears quite different from ordinary decomposition. At
the Galapagos Archipelago (as will be described in a future chapter),
great beds are formed of volcanic ashes and particles of scoriae, which
have undergone a closely similar change.

THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.

(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.)

A.—Ancient volcanic rocks.

B.—Calcareous stratum.

C.—Upper basaltic lava.)

The upper line of surface of the calcareous stratum, which is so
conspicuous from being quite white and so nearly horizontal, ranges for
miles along the coast, at the height of about sixty feet above the sea.
The sheet of basalt, by which it is capped, is on an average eighty
feet in thickness. Westward of Porto Praya beyond Red Hill, the white
stratum with the superincumbent basalt is covered up by more recent
streams. Northward of Signal Post Hill, I could follow it with my eye,
trending away for several miles along the sea cliffs. The distance thus
observed is about seven miles; but I cannot doubt from its regularity
that it extends much farther. In some ravines at right angles to the
coast, it is seen gently dipping towards the sea, probably with the
same inclination as when deposited round the ancient shores of the
island. I found only one inland section, namely, at the base of the
hill marked A, where, at the height of some hundred feet, this bed was
exposed; it here rested on the usual compact augitic rock associated
with wacke, and was covered by the widespread sheet of modern basaltic
lava. Some exceptions occur to the horizontality of the white stratum:
at Quail Island, its upper surface is only forty feet above the level
of the sea; here also the capping of lava is only between twelve and
fifteen feet in thickness; on the other hand, at the north-east side of
Porto Praya harbour, the calcareous stratum, as well as the rock on
which it rests, attain a height above the average level: the inequality
of level in these two cases is not, as I believe, owing to unequal
elevation, but to original irregularities at the bottom of the sea. Of
this fact, at Quail Island, there was clear evidence in the calcareous
deposit being in one part of much greater than the average thickness,
and in another part being entirely absent; in this latter case, the
modern basaltic lavas rested directly on those of more ancient origin.

Under Signal Post Hill, the white stratum dips into the sea in a
remarkable manner. This hill is conical, 450 feet in height, and
retains some traces of having had a crateriform structure; it is
composed chiefly of matter erupted posteriorly to the elevation of the
great basaltic plain, but partly of lava of apparently submarine origin
and of considerable antiquity. The surrounding plain, as well as the
eastern flank of this hill, has been worn into steep precipices,
overhanging the sea. In these precipices, the white calcareous stratum
may be seen, at the height of about seventy feet above the beach,
running for some miles both northward and southward of the hill, in a
line appearing to be perfectly horizontal; but for a space of a quarter
of a mile directly under the hill, it dips into the sea and disappears.
On the south side the dip is gradual, on the north side it is more
abrupt, as is shown in Figure 2. As neither the calcareous stratum, nor
the superincumbent basaltic lava (as far as the latter can be
distinguished from the more modern ejections), appears to thicken as it
dips, I infer that these strata were not originally accumulated in a
trough, the centre of which afterwards became a point of eruption; but
that they have subsequently been disturbed and bent. We may suppose
either that Signal Post Hill subsided after its elevation with the
surrounding country, or that it never was uplifted to the same height
with it. This latter seems to me the most probable alternative, for
during the slow and equable elevation of this portion of the island,
the subterranean motive power, from expending part of its force in
repeatedly erupting volcanic matter from beneath this point, would, it
is likely, have less force to uplift it. Something of the same kind
seems to have occurred near Red Hill, for when tracing upwards the
naked streams of lava from near Porto Praya towards the interior of the
island, I was strongly induced to suspect, that since the lava had
flowed, the slope of the land had been slightly modified, either by a
small subsidence near Red Hill, or by that portion of the plain having
been uplifted to a less height during the elevation of the whole area.

THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT.

This lava is of a pale grey colour, fusing into a black enamel; its
fracture is rather earthy and concretionary; it contains olivine in
small grains. The central parts of the mass are compact, or at most
crenulated with a few minute cavities, and are often columnar. At Quail
Island this structure was assumed in a striking manner; the lava in one
part being divided into horizontal laminae, which became in another
part split by vertical fissures into five-sided plates; and these
again, being piled on each other, insensibly became soldered together,
forming fine symmetrical columns. The lower surface of the lava is
vesicular, but sometimes only to the thickness of a few inches; the
upper surface, which is likewise vesicular, is divided into balls,
frequently as much as three feet in diameter, made up of concentric
layers. The mass is composed of more than one stream; its total
thickness being, on an average, about eighty feet: the lower portion
has certainly flowed beneath the sea, and probably likewise the upper
portion. The chief part of this lava has flowed from the central
districts, between the hills marked A, B, C, etc., in the woodcut- map.
The surface of the country, near the coast, is level and barren;
towards the interior, the land rises by successive terraces, of which
four, when viewed from a distance, could be distinctly counted.

VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE
EJECTED MATTER ASSOCIATED WITH EARTHY LIME.

These recent lavas have proceeded from those scattered, conical,
reddish- coloured hills, which rise abruptly from the plain-country
near the coast. I ascended some of them, but will describe only one,
namely, RED HILL, which may serve as a type of its class, and is
remarkable in some especial respects. Its height is about six hundred
feet; it is composed of bright red, highly scoriaceous rock of a
basaltic nature; on one side of its summit there is a hollow, probably
the last remnant of a crater. Several of the other hills of this class,
judging from their external forms, are surmounted by much more perfect
craters. When sailing along the coast, it was evident that a
considerable body of lava had flowed from Red Hill, over a line of
cliff about one hundred and twenty feet in height, into the sea: this
line of cliff is continuous with that forming the coast, and bounding
the plain on both sides of this hill; these streams, therefore, were
erupted, after the formation of the coast-cliffs, from Red Hill, when
it must have stood, as it now does, above the level of the sea. This
conclusion accords with the highly scoriaceous condition of all the
rock on it, appearing to be of subaerial formation: and this is
important, as there are some beds of calcareous matter near its summit,
which might, at a hasty glance, have been mistaken for a submarine
deposit. These beds consist of white, earthy, carbonate of lime,
extremely friable so as to be crushed with the least pressure; the most
compact specimens not resisting the strength of the fingers. Some of
the masses are as white as quicklime, and appear absolutely pure; but
on examining them with a lens, minute particles of scoriae can always
be seen, and I could find none which, when dissolved in acids, did not
leave a residue of this nature. It is, moreover, difficult to find a
particle of the lime which does not change colour under the blowpipe,
most of them even becoming glazed. The scoriaceous fragments and the
calcareous matter are associated in the most irregular manner,
sometimes in obscure beds, but more generally as a confused breccia,
the lime in some parts and the scoriae in others being most abundant.
Sir H. De la Beche has been so kind as to have some of the purest
specimens analysed, with a view to discover, considering their volcanic
origin, whether they contained much magnesia; but only a small portion
was found, such as is present in most limestones.

Fragments of the scoriae embedded in the calcareous mass, when broken,
exhibit many of their cells lined and partly filled with a white,
delicate, excessively fragile, moss-like, or rather conferva-like,
reticulation of carbonate of lime. These fibres, examined under a lens
of one-tenth of an inch focal distance, appear cylindrical; they are
rather above one- thousandth of an inch in diameter; they are either
simply branched, or more commonly united into an irregular mass of
network, with the meshes of very unequal sizes and of unequal numbers
of sides. Some of the fibres are thickly covered with extremely minute
spicula, occasionally aggregated into little tuffs; and hence they have
a hairy appearance. These spicula are of the same diameter throughout
their length; they are easily detached, so that the object-glass of the
microscope soon becomes scattered over with them. Within the cells of
many fragments of the scoria, the lime exhibits this fibrous structure,
but generally in a less perfect degree. These cells do not appear to be
connected with one another. There can be no doubt, as will presently be
shown, that the lime was erupted, mingled with the lava in its fluid
state, and therefore I have thought it worth while to describe minutely
this curious fibrous structure, of which I know nothing analogous. From
the earthy condition of the fibres, this structure does not appear to
be related to crystallisation.

Other fragments of the scoriaceous rock from this hill, when broken,
are often seen marked with short and irregular white streaks, which are
owing to a row of separate cells being partly, or quite, filled with
white calcareous powder. This structure immediately reminded me of the
appearance in badly kneaded dough, of balls and drawn-out streaks of
flour, which have remained unmixed with the paste; and I cannot doubt
that small masses of the lime, in the same manner remaining unmixed
with the fluid lava, have been drawn out when the whole was in motion.
I carefully examined, by trituration and solution in acids, pieces of
the scoriae, taken from within half-an-inch of those cells which were
filled with the calcareous powder, and they did not contain an atom of
free lime. It is obvious that the lava and lime have on a large scale
been very imperfectly mingled; and where small portions of the lime
have been entangled within a piece of the viscid lava, the cause of
their now occupying, in the form of a powder or of a fibrous
reticulation, the vesicular cavities, is, I think, evidently due to the
confined gases having most readily expanded at the points where the
incoherent lime rendered the lava less adhesive.

A mile eastward of the town of Praya, there is a steep-sided gorge,
about one hundred and fifty yards in width, cutting through the
basaltic plain and underlying beds, but since filled up by a stream of
more modern lava. This lava is dark grey, and in most parts compact and
rudely columnar; but at a little distance from the coast, it includes
in an irregular manner a brecciated mass of red scoriae mingled with a
considerable quantity of white, friable, and in some parts, nearly pure
earthy lime, like that on the summit of Red Hill. This lava, with its
entangled lime, has certainly flowed in the form of a regular stream;
and, judging from the shape of the gorge, towards which the drainage of
the country (feeble though it now be) still is directed, and from the
appearance of the bed of loose water-worn blocks with their interstices
unfilled, like those in the bed of a torrent, on which the lava rests,
we may conclude that the stream was of subaerial origin. I was unable
to trace it to its source, but, from its direction, it seemed to have
come from Signal Post Hill, distant one mile and a quarter, which, like
Red Hill, has been a point of eruption subsequent to the elevation of
the great basaltic plain. It accords with this view, that I found on
Signal Post Hill, a mass of earthy, calcareous matter of the same
nature, mingled with scoriae. I may here observe that part of the
calcareous matter forming the horizontal sedimentary bed, especially
the finer matter with which the embedded fragments of rock are
whitewashed, has probably been derived from similar volcanic eruptions,
as well as from triturated organic remains: the underlying, ancient,
crystalline rocks, also, are associated with much carbonate of lime,
filling amygdaloidal cavities, and forming irregular masses, the nature
of which latter I was unable to understand.

Considering the abundance of earthy lime near the summit of Red Hill, a
volcanic cone six hundred feet in height, of subaerial
growth,—considering the intimate manner in which minute particles and
large masses of scoriae are embedded in the masses of nearly pure lime,
and on the other hand, the manner in which small kernels and streaks of
the calcareous powder are included in solid pieces of the
scoriae,—considering, also, the similar occurrence of lime and scoriae
within a stream of lava, also supposed, with good reason, to have been
of modern subaerial origin, and to have flowed from a hill, where
earthy lime also occurs: I think, considering these facts, there can be
no doubt that the lime has been erupted, mingled with the molten lava.
I am not aware that any similar case has been described: it appears to
me an interesting one, inasmuch as most geologists must have speculated
on the probable effects of a volcanic focus, bursting through
deep-seated beds of different mineralogical composition. The great
abundance of free silex in the trachytes of some countries (as
described by Beudant in Hungary, and by P. Scrope in the Panza
Islands), perhaps solves the inquiry with respect to deep-seated beds
of quartz; and we probably here see it answered, where the volcanic
action has invaded subjacent masses of limestone. One is naturally led
to conjecture in what state the now earthy carbonate of lime existed,
when ejected with the intensely heated lava: from the extreme
cellularity of the scoriae on Red Hill, the pressure cannot have been
great, and as most volcanic eruptions are accompanied by the emission
of large quantities of steam and other gases, we here have the most
favourable conditions, according to the views at present entertained by
chemists, for the expulsion of the carbonic acid. (Whilst deep beneath
the surface, the carbonate of lime was, I presume, in a fluid state.
Hutton, it is known, thought that all amygdaloids were produced by
drops of molten limestone floating in the trap, like oil in water: this
no doubt is erroneous, but if the matter forming the summit of Red Hill
had been cooled under the pressure of a moderately deep sea, or within
the walls of a dike, we should, in all probability, have had a trap
rock associated with large masses of compact, crystalline, calcareous
spar, which, according to the views entertained by many geologists,
would have been wrongly attributed to subsequent infiltration.) Has the
slow re- absorption of this gas, it may be asked, given to the lime in
the cells of the lava, that peculiar fibrous structure, like that of an
efflorescing salt? Finally, I may remark on the great contrast in
appearance between this earthy lime, which must have been heated in a
free atmosphere of steam and other gases, while the white, crystalline,
calcareous spar, produced by a single thin sheet of lava (as at Quail
Island) rolling over similar earthy lime and the debris of organic
remains, at the bottom of a shallow sea.

SIGNAL POST HILL.

This hill has already been several times mentioned, especially with
reference to the remarkable manner in which the white calcareous
stratum, in other parts so horizontal (Figure 2), dips under it into
the sea. It has a broad summit, with obscure traces of a crateriform
structure, and is composed of basaltic rocks (Of these, one common
variety is remarkable for being full of small fragments of a dark
jasper-red earthy mineral, which, when examined carefully, shows an
indistinct cleavage; the little fragments are elongated in form, are
soft, are magnetic before and after being heated, and fuse with
difficulty into a dull enamel. This mineral is evidently closely
related to the oxides of iron, but I cannot ascertain what it exactly
is. The rock containing this mineral is crenulated with small angular
cavities, which are lined and filled with yellowish crystals of
carbonate of lime.), some compact, others highly cellular with inclined
beds of loose scoriae, of which some are associated with earthy lime.
Like Red Hill, it has been the source of eruptions, subsequently to the
elevation of the surrounding basaltic plain; but unlike that hill, it
has undergone considerable denudation, and has been the seat of
volcanic action at a remote period, when beneath the sea. I judge of
this latter circumstance from finding on its inland flank the last
remains of three small points of eruption. These points are composed of
glossy scoriae, cemented by crystalline calcareous spar, exactly like
the great submarine calcareous deposit, where the heated lava has
rolled over it: their demolished state can, I think, be explained only
by the denuding action of the waves of the sea. I was guided to the
first orifice by observing a sheet of lava, about two hundred yards
square, with steepish sides, superimposed on the basaltic plain with no
adjoining hillock, whence it could have been erupted; and the only
trace of a crater which I was able to discover, consisted of some
inclined beds of scoriae at one of its corners. At the distance of
fifty yards from a second level-topped patch of lava, but of much
smaller size, I found an irregular circular group of masses of
cemented, scoriaceous breccia, about six feet in height, which
doubtless had once formed the point of eruption. The third orifice is
now marked only by an irregular circle of cemented scoriae, about four
yards in diameter, and rising in its highest point scarcely three feet
above the level of the plain, the surface of which, close all round,
exhibits its usual appearance: here we have a horizontal basal section
of a volcanic spiracle, which, together with all its ejected matter,
has been almost totally obliterated.

The stream of lava, which fills the narrow gorge eastward of the town
of Praya, judging from its course, seems, as before remarked, to have
come from Signal Post Hill, and to have flowed over the plain, after
its elevation (The sides of this gorge, where the upper basaltic
stratum is intersected, are almost perpendicular. The lava, which has
since filled it up, is attached to these sides, almost as firmly as a
dike is to its walls. In most cases, where a stream of lava has flowed
down a valley, it is bounded on each side by loose scoriaceous
masses.): the same observation applies to a stream (possibly part of
the same one) capping the sea cliffs, a little eastward of the gorge.
When I endeavoured to follow these streams over the stony level plain,
which is almost destitute of soil and vegetation, I was much surprised
to find, that although composed of hard basaltic matter, and not having
been exposed to marine denudation, all distant traces of them soon
became utterly lost. But I have since observed at the Galapagos
Archipelago, that it is often impossible to follow even great deluges
of quite recent lava across older streams, except by the size of the
bushes growing on them, or by the comparative states of glossiness of
their surfaces,—characters which a short lapse of time would be
sufficient quite to obscure. I may remark, that in a level country,
with a dry climate, and with the wind blowing always in one direction
(as at the Cape de Verde Archipelago), the effects of atmospheric
degradation are probably much greater than would at first be expected;
for soil in this case accumulates only in a few protected hollows, and
being blown in one direction, it is always travelling towards the sea
in the form of the finest dust, leaving the surface of the rocks bare,
and exposed to the full effects of renewed meteoric action.

INLAND HILLS OF MORE ANCIENT VOLCANIC ROCKS.

These hills are laid down by eye, and marked as A, B, C, etc., in Map
1. They are related in mineralogical composition, and are probably
directly continuous with the lowest rocks exposed on the coast. These
hills, viewed from a distance, appear as if they had once formed part
of an irregular tableland, and from their corresponding structure and
composition this probably has been the case. They have flat, slightly
inclined summits, and are, on an average, about six hundred feet in
height; they present their steepest slope towards the interior of the
island, from which point they radiate outwards, and are separated from
each other by broad and deep valleys, through which the great streams
of lava, forming the coast-plains, have descended. Their inner and
steeper escarpments are ranged in an irregular curve, which rudely
follows the line of the shore, two or three miles inland from it. I
ascended a few of these hills, and from others, which I was able to
examine with a telescope, I obtained specimens, through the kindness of
Mr. Kent, the assistant-surgeon of the “Beagle”; although by these
means I am acquainted with only a part of the range, five or six miles
in length, yet I scarcely hesitate, from their uniform structure, to
affirm that they are parts of one great formation, stretching round
much of the circumference of the island.

The upper and lower strata of these hills differ greatly in
composition. The upper are basaltic, generally compact, but sometimes
scoriaceous and amygdaloidal, with associated masses of wacke: where
the basalt is compact, it is either fine-grained or very coarsely
crystallised; in the latter case it passes into an augitic rock,
containing much olivine; the olivine is either colourless, or of the
usual yellow and dull reddish shades. On some of the hills, beds of
calcareous matter, both in an earthy and in a crystalline form,
including fragments of glossy scoriae, are associated with the basaltic
strata. These strata differ from the streams of basaltic lava forming
the coast-plains, only in being more compact, and in the crystals of
augite, and in the grains of olivine being of much greater
size;—characters which, together with the appearance of the associated
calcareous beds, induce me to believe that they are of submarine
formation.

Some considerable masses of wacke, which are associated with these
basaltic strata, and which likewise occur in the basal series on the
coast, especially at Quail Island, are curious. They consist of a pale
yellowish- green argillaceous substance, of a crumbling texture when
dry, but unctuous when moist: in its purest form, it is of a beautiful
green tint, with translucent edges, and occasionally with obscure
traces of an original cleavage. Under the blowpipe it fuses very
readily into a dark grey, and sometimes even black bead, which is
slightly magnetic. From these characters, I naturally thought that it
was one of the pale species, decomposed, of the genus augite;—a
conclusion supported by the unaltered rock being full of large separate
crystals of black augite, and of balls and irregular streaks of dark
grey augitic rock. As the basalt ordinarily consists of augite, and of
olivine often tarnished and of a dull red colour, I was led to examine
the stages of decomposition of this latter mineral, and I found, to my
surprise, that I could trace a nearly perfect gradation from unaltered
olivine to the green wacke. Part of the same grain under the blowpipe
would in some instances behave like olivine, its colour being only
slightly changed, and part would give a black magnetic bead. Hence I
can have no doubt that the greenish wacke originally existed as
olivine; but great chemical changes must have been effected during the
act of decomposition thus to have altered a very hard, transparent,
infusible mineral, into a soft, unctuous, easily melted, argillaceous
substance. (D’Aubuisson “Traite de Geognosie” tome 2 page 569 mentions,
on the authority of M. Marcel de Serres, masses of green earth near
Montpellier, which are supposed to be due to the decomposition of
olivine. I do not, however, find, that the action of this mineral under
the blowpipe being entirely altered, as it becomes decomposed, has been
noticed; and the knowledge of this fact is important, as at first it
appears highly improbable that a hard, transparent, refractory mineral
should be changed into a soft, easily fused clay, like this of St.
Jago. I shall hereafter describe a green substance, forming threads
within the cells of some vesicular basaltic rocks in Van Diemen’s Land,
which behave under the blowpipe like the green wacke of St. Jago; but
its occurrence in cylindrical threads, shows it cannot have resulted
from the decomposition of olivine, a mineral always existing in the
form of grains or crystals.)

The basal strata of these hills, as well as some neighbouring,
separate, bare, rounded hillocks, consist of compact, fine-grained,
non-crystalline (or so slightly as scarcely to be perceptible),
ferruginous, feldspathic rocks, and generally in a state of
semi-decomposition. Their fracture is exceedingly irregular, and
splintery; yet small fragments are often very tough. They contain much
ferruginous matter, either in the form of minute grains with a metallic
lustre, or of brown hair-like threads: the rock in this latter case
assuming a pseudo-brecciated structure. These rocks sometimes contain
mica and veins of agate. Their rusty brown or yellowish colour is
partly due to the oxides of iron, but chiefly to innumerable,
microscopically minute, black specks, which, when a fragment is heated,
are easily fused, and evidently are either hornblende or augite. These
rocks, therefore, although at first appearing like baked clay or some
altered sedimentary deposit, contain all the essential ingredients of
trachyte; from which they differ only in not being harsh, and in not
containing crystals of glassy feldspar. As is so often the case with
trachytic formation, no stratification is here apparent. A person would
not readily believe that these rocks could have flowed as lava; yet at
St. Helena there are well-characterised streams (as will be described
in an ensuing chapter) of nearly similar composition. Amidst the
hillocks composed of these rocks, I found in three places, smooth
conical hills of phonolite, abounding with fine crystals of glassy
feldspar, and with needles of hornblende. These cones of phonolite, I
believe, bear the same relation to the surrounding feldspathic strata
which some masses of coarsely crystallised augitic rock, in another
part of the island, bear to the surrounding basalt, namely, that both
have been injected. The rocks of a feldspathic nature being anterior in
origin to the basaltic strata, which cap them, as well as to the
basaltic streams of the coast-plains, accords with the usual order of
succession of these two grand divisions of the volcanic series.

The strata of most of these hills in the upper part, where alone the
planes of division are distinguishable, are inclined at a small angle
from the interior of the island towards the sea-coast. The inclination
is not the same in each hill; in that marked A it is less than in B, D,
or E; in C the strata are scarcely deflected from a horizontal plane,
and in F (as far as I could judge without ascending it) they are
slightly inclined in a reverse direction, that is, inwards and towards
the centre of the island. Notwithstanding these differences of
inclination, their correspondence in external form, and in the
composition both of their upper and lower parts,- -their relative
position in one curved line, with their steepest sides turned
inwards,—all seem to show that they originally formed parts of one
platform; which platform, as before remarked, probably extended round a
considerable portion of the circumference of the island. The upper
strata certainly flowed as lava, and probably beneath the sea, as
perhaps did the lower feldspathic masses: how then come these strata to
hold their present position, and whence were they erupted?

In the centre of the island there are lofty mountains, but they are
separated from the steep inland flanks of these hills by a wide space
of lower country: the interior mountains, moreover, seem to have been
the source of those great streams of basaltic lava which, contracting
as they pass between the bases of the hills in question, expand into
the coast- plains. (I saw very little of the inland parts of the
island. Near the village of St. Domingo, there are magnificent cliffs
of rather coarsely crystallised basaltic lava. Following the little
stream in this valley, about a mile above the village, the base of the
great cliff was formed of a compact fine-grained basalt, conformably
covered by a bed of pebbles. Near Fuentes, I met with pap-formed hills
of the compact feldspathic series of rocks.) Round the shores of St.
Helena there is a rudely formed ring of basaltic rocks, and at
Mauritius there are remnants of another such a ring round part, if not
round the whole, of the island; here again the same question
immediately occurs, how came these masses to hold their present
position, and whence were they erupted? The same answer, whatever it
may be, probably applies in these three cases; and in a future chapter
we shall recur to this subject.

VALLEYS NEAR THE COAST.

These are broad, very flat, and generally bounded by low cliff-formed
sides. Portions of the basaltic plain are sometimes nearly or quite
isolated by them; of which fact, the space on which the town of Praya
stands offers an instance. The great valley west of the town has its
bottom filled up to a depth of more than twenty feet by well-rounded
pebbles, which in some parts are firmly cemented together by white
calcareous matter. There can be no doubt, from the form of these
valleys, that they were scooped out by the waves of the sea, during
that equable elevation of the land, of which the horizontal calcareous
deposit, with its existing species of marine remains, gives evidence.
Considering how well shells have been preserved in this stratum, it is
singular that I could not find even a single small fragment of shell in
the conglomerate at the bottom of the valleys. The bed of pebbles in
the valley west of the town is intersected by a second valley joining
it as a tributary, but even this valley appears much too wide and
flat-bottomed to have been formed by the small quantity of water, which
falls only during one short wet season; for at other times of the year
these valleys are absolutely dry.

RECENT CONGLOMERATE.

On the shores of Quail Island, I found fragments of brick, bolts of
iron, pebbles, and large fragments of basalt, united by a scanty base
of impure calcareous matter into a firm conglomerate. To show how
exceedingly firm this recent conglomerate is, I may mention, that I
endeavoured with a heavy geological hammer to knock out a thick bolt of
iron, which was embedded a little above low-water mark, but was quite
unable to succeed.




CHAPTER II.
FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.


FERNANDO NORONHA. Precipitous hill of phonolite.

TERCEIRA. Trachytic rocks: their singular decomposition by steam of
high temperature.

TAHITI. Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype.

MAURITIUS. Proofs of its recent elevation. Structure of its more
ancient mountains; similarity with St. Jago.

ST. PAUL’S ROCKS. Not of volcanic origin. Their singular mineralogical
composition.

FERNANDO NORONHA.

During our short visit at this and the four following islands, I
observed very little worthy of description. Fernando Noronha is
situated in the Atlantic Ocean, in latitude 3 degrees 50 minutes S.,
and 230 miles distant from the coast of South America. It consists of
several islets, together nine miles in length by three in breadth. The
whole seems to be of volcanic origin; although there is no appearance
of any crater, or of any one central eminence. The most remarkable
feature is a hill 1,000 feet high, of which the upper 400 feet consist
of a precipitous, singularly shaped pinnacle, formed of columnar
phonolite, containing numerous crystals of glassy feldspar, and a few
needles of hornblende. From the highest accessible point of this hill,
I could distinguish in different parts of the group several other
conical hills, apparently of the same nature. At St. Helena there are
similar, great, conical, protuberant masses of phonolite, nearly one
thousand feet in height, which have been formed by the injection of
fluid feldspathic lava into yielding strata. If this hill has had, as
is probable, a similar origin, denudation has been here effected on an
enormous scale. Near the base of this hill, I observed beds of white
tuff, intersected by numerous dikes, some of amygdaloidal basalt and
others of trachyte; and beds of slaty phonolite with the planes of
cleavage directed N.W. and S.E. Parts of this rock, where the crystals
were scanty, closely resembled common clay-slate, altered by the
contact of a trap-dike. The lamination of rocks, which undoubtedly have
once been fluid, appears to me a subject well deserving attention. On
the beach there were numerous fragments of compact basalt, of which
rock a distant facade of columns seemed to be formed.

TERCEIRA IN THE AZORES.

The central parts of this island consist of irregularly rounded
mountains of no great elevation, composed of trachyte, which closely
resembles in general character the trachyte of Ascension, presently to
be described. This formation is in many parts overlaid, in the usual
order of superposition, by streams of basaltic lava, which near the
coast compose nearly the whole surface. The course which these streams
have followed from their craters, can often be followed by the eye. The
town of Angra is overlooked by a crateriform hill (Mount Brazil),
entirely built of thin strata of fine-grained, harsh, brown-coloured
tuff. The upper beds are seen to overlap the basaltic streams on which
the town stands. This hill is almost identical in structure and
composition with numerous crateriformed hills in the Galapagos
Archipelago.

EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.

In the central part of the island there is a spot, where steam is
constantly issuing in jets from the bottom of a small ravine-like
hollow, which has no exit, and which abuts against a range of trachytic
mountains. The steam is emitted from several irregular fissures: it is
scentless, soon blackens iron, and is of much too high temperature to
be endured by the hand. The manner in which the solid trachyte is
changed on the borders of these orifices is curious: first, the base
becomes earthy, with red freckles evidently due to the oxidation of
particles of iron; then it becomes soft; and lastly, even the crystals
of glassy feldspar yield to the dissolving agent. After the mass is
converted into clay, the oxide of iron seems to be entirely removed
from some parts, which are left perfectly white, whilst in other
neighbouring parts, which are of the brightest red colour, it seems to
be deposited in greater quantity; some other masses are marbled with
two distinct colours. Portions of the white clay, now that they are
dry, cannot be distinguished by the eye from the finest prepared chalk;
and when placed between the teeth they feel equally soft-grained; the
inhabitants use this substance for white-washing their houses. The
cause of the iron being dissolved in one part, and close by being again
deposited, is obscure; but the fact has been observed in several other
places. (Spallanzani, Dolomieu, and Hoffman have described similar
cases in the Italian volcanic islands. Dolomieu says the iron at the
Panza Islands is redeposited in the form of veins (page 86 “Memoire sur
les Isles Ponces”). These authors likewise believe that the steam
deposits silica: it is now experimentally known that vapour of a high
temperature is able to dissolve silica.) In some half-decayed
specimens, I found small, globular aggregations of yellow hyalite,
resembling gum-arabic, which no doubt had been deposited by the steam.

As there is no escape for the rain-water, which trickles down the sides
of the ravine-like hollow, whence the steam issues, it must all
percolate downwards through the fissures at its bottom. Some of the
inhabitants informed me that it was on record that flames (some
luminous appearance?) had originally proceeded from these cracks, and
that the flames had been succeeded by the steam; but I was not able to
ascertain how long this was ago, or anything certain on the subject.
When viewing the spot, I imagined that the injection of a large mass of
rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid
state, by arching the surface might have caused a wedge-shaped hollow
with cracks at the bottom, and that the rain- water percolating to the
neighbourhood of the heated mass, would during many succeeding years be
driven back in the form of steam.

TAHITI (OTAHEITE).

I visited only a part of the north-western side of this island, and
this part is entirely composed of volcanic rocks. Near the coast there
are several varieties of basalt, some abounding with large crystals of
augite and tarnished olivine, others compact and earthy,—some slightly
vesicular, and others occasionally amygdaloidal. These rocks are
generally much decomposed, and to my surprise, I found in several
sections that it was impossible to distinguish, even approximately, the
line of separation between the decayed lava and the alternating beds of
tuff. Since the specimens have become dry, it is rather more easy to
distinguish the decomposed igneous rocks from the sedimentary tuffs.
This gradation in character between rocks having such widely different
origins, may I think be explained by the yielding under pressure of the
softened sides of the vesicular cavities, which in many volcanic rocks
occupy a large proportion of their bulk. As the vesicles generally
increase in size and number in the upper parts of a stream of lava, so
would the effects of their compression increase; the yielding,
moreover, of each lower vesicle must tend to disturb all the softened
matter above it. Hence we might expect to trace a perfect gradation
from an unaltered crystalline rock to one in which all the particles
(although originally forming part of the same solid mass) had undergone
mechanical displacement; and such particles could hardly be
distinguished from others of similar composition, which had been
deposited as sediment. As lavas are sometimes laminated in their upper
parts even horizontal lines, appearing like those of aqueous
deposition, could not in all cases be relied on as a criterion of
sedimentary origin. From these considerations it is not surprising that
formerly many geologists believed in real transitions from aqueous
deposits, through wacke, into igneous traps.

In the valley of Tia-auru, the commonest rocks are basalts with much
olivine, and in some cases almost composed of large crystals of augite.
I picked up some specimens, with much glassy feldspar, approaching in
character to trachyte. There were also many large blocks of vesicular
basalt, with the cavities beautifully lined with chabasie (?), and
radiating bundles of mesotype. Some of these specimens presented a
curious appearance, owing to a number of the vesicles being half filled
up with a white, soft, earthy mesotypic mineral, which intumesced under
the blowpipe in a remarkable manner. As the upper surfaces in all the
half-filled cells are exactly parallel, it is evident that this
substance has sunk to the bottom of each cell from its weight.
Sometimes, however, it entirely fills the cells. Other cells are either
quite filled, or lined, with small crystals, apparently of chabasie;
these crystals, also, frequently line the upper half of the cells
partly filled with the earthy mineral, as well as the upper surface of
this substance itself, in which case the two minerals appear to blend
into each other. I have never seen any other amygdaloid with the cells
half filled in the manner here described; and it is difficult to
imagine the causes which determined the earthy mineral to sink from its
gravity to the bottom of the cells, and the crystalline mineral to
adhere in a coating of equal thickness round the sides of the cells.
(MacCulloch, however, has described and given a plate of (“Geolog.
Trans.” 1st series volume 4 page 225) a trap rock, with cavities filled
up horizontally with quartz and chalcedony. The upper halves of these
cavities are often filled by layers, which follow each irregularity of
the surface, and by little depending stalactites of the same siliceous
substances.)

The basic strata on the sides of the valley are gently inclined
seaward, and I nowhere observed any sign of disturbance; the strata are
separated from each other by thick, compact beds of conglomerate, in
which the fragments are large, some being rounded, but most angular.
From the character of these beds, from the compact and crystalline
condition of most of the lavas, and from the nature of the infiltrated
minerals, I was led to conjecture that they had originally flowed
beneath the sea. This conclusion agrees with the fact that the Rev. W.
Ellis found marine remains at a considerable height, which he believes
were interstratified with volcanic matter; as is likewise described to
be the case by Messrs. Tyerman and Bennett at Huaheine, an island in
this same archipelago. Mr. Stutchbury also discovered near the summit
of one of the loftiest mountains of Tahiti, at the height of several
thousand feet, a stratum of semi-fossil coral. None of these remains
have been specifically examined. On the coast, where masses of
coral-rock would have afforded the clearest evidence, I looked in vain
for any signs of recent elevation. For references to the above
authorities, and for more detailed reasons for not believing that
Tahiti has been recently elevated, I must refer to the “Structure and
Distribution of Coral-Reefs.”

MAURITIUS.

Approaching this island on the northern or north-western side, a curved
chain of bold mountains, surmounted by rugged pinnacles, is seen to
rise from a smooth border of cultivated land, which gently slopes down
to the coast. At the first glance, one is tempted to believe that the
sea lately reached the base of these mountains, and upon examination,
this view, at least with respect to the inferior parts of the border,
is found to be perfectly correct. Several authors have described masses
of upraised coral- rock round the greater part of the circumference of
the island. (Captain Carmichael, in Hooker’s “Bot. Misc.” volume 2 page
301. Captain Lloyd has lately, in the “Proceedings of the Geological
Society” (volume 3 page 317), described carefully some of these masses.
In the “Voyage a l’Isle de France, par un Officier du Roi,” many
interesting facts are given on this subject. Consult also “Voyage aux
Quatre Isles d’Afrique, par M. Bory St. Vincent.”) Between Tamarin Bay
and the Great Black River I observed, in company with Captain Lloyd,
two hillocks of coral-rock, formed in their lower part of hard
calcareous sandstone, and in their upper of great blocks, slightly
aggregated, of Astraea and Madrepora, and of fragments of basalt; they
were divided into beds dipping seaward, in one case at an angle of 8
degrees, and in the other at 18 degrees; they had a water-worn
appearance, and they rose abruptly from a smooth surface, strewed with
rolled debris of organic remains, to a height of about twenty feet. The
Officier du Roi, in his most interesting tour in 1768 round the island,
has described masses of upraised coral-rocks, still retaining that
moat-like structure (see my “Coral Reefs”) which is characteristic of
the living reefs. On the coast northward of Port Louis, I found the
lava concealed for a considerable space inland by a conglomerate of
corals and shells, like those on the beach, but in parts consolidated
by red ferruginous matter. M. Bory St. Vincent has described similar
calcareous beds over nearly the whole of the plain of Pamplemousses.
Near Port Louis, when turning over some large stones, which lay in the
bed of a stream at the head of a protected creek, and at the height of
some yards above the level of spring tides, I found several shells of
serpula still adhering to their under sides.

The jagged mountains near Port Louis rise to a height of between two
and three thousand feet; they consist of strata of basalt, obscurely
separated from each other by firmly aggregated beds of fragmentary
matter; and they are intersected by a few vertical dikes. The basalt in
some parts abounds with large crystals of augite and olivine, and is
generally compact. The interior of the island forms a plain, raised
probably about a thousand feet above the level of the sea, and composed
of streams of lava which have flowed round and between the rugged
basaltic mountains. These more recent lavas are also basaltic, but less
compact, and some of them abound with feldspar, so that they even fuse
into a pale coloured glass. On the banks of the Great River, a section
is exposed nearly five hundred feet deep, worn through numerous thin
sheets of the lava of this series, which are separated from each other
by beds of scoriae. They seem to have been of subaerial formation, and
to have flowed from several points of eruption on the central platform,
of which the Piton du Milieu is said to be the principal one. There are
also several volcanic cones, apparently of this modern period, round
the circumference of the island, especially at the northern end, where
they form separate islets.

The mountains composed of the more compact and crystalline basalt, form
the main skeleton of the island. M. Bailly (“Voyage aux Terres
Australes” tome 1 page 54.) states that they all “se developpent autour
d’elle comme une ceinture d’immenses remparts, toutes affectant une
pente plus ou moins enclinee vers le rivage de la mer; tandis, au
contraire, que vers le centre de l’ile elles presentent une coupe
abrupte, et souvent taillee a pic. Toutes ces montagnes sont formees de
couches paralleles inclinees du centre de l’ile vers la mer.” These
statements have been disputed, though not in detail, by M. Quoy, in the
voyage of Freycinet. As far as my limited means of observation went, I
found them perfectly correct. (M. Lesson, in his account of this
island, in the “Voyage of the ‘Coquille’,” seems to follow M. Bailly’s
views.) The mountains on the N.W. side of the island, which I examined,
namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and
apparently another farther southward, have precisely the external shape
and stratification described by M. Bailly. They form about a quarter of
his girdle of ramparts. Although these mountains now stand quite
detached, being separated from each other by breaches, even several
miles in width, through which deluges of lava have flowed from the
interior of the island; nevertheless, seeing their close general
similarity, one must feel convinced that they originally formed parts
of one continuous mass. Judging from the beautiful map of the
Mauritius, published by the Admiralty from a French MS., there is a
range of mountains (M. Bamboo) on the opposite side of the island,
which correspond in height, relative position, and external form, with
those just described. Whether the girdle was ever complete may well be
doubted; but from M. Bailly’s statements, and my own observations, it
may be safely concluded that mountains with precipitous inland flanks,
and composed of strata dipping outwards, once extended round a
considerable portion of the circumference of the island. The ring
appears to have been oval and of vast size; its shorter axis, measured
across from the inner sides of the mountains near Port Louis and those
near Grand Port, being no less than thirteen geographical miles in
length. M. Bailly boldly supposes that this enormous gulf, which has
since been filled up to a great extent by streams of modern lava, was
formed by the sinking in of the whole upper part of one great volcano.

It is singular in how many respects those portions of St. Jago and of
Mauritius which I visited agree in their geological history. At both
islands, mountains of similar external form, stratification, and (at
least in their upper beds) composition, follow in a curved chain the
coast-line. These mountains in each case appear originally to have
formed parts of one continuous mass. The basaltic strata of which they
are composed, from their compact and crystalline structure, seem, when
contrasted with the neighbouring basaltic streams of subaerial
formation, to have flowed beneath the pressure of the sea, and to have
been subsequently elevated. We may suppose that the wide breaches
between the mountains were in both cases worn by the waves, during
their gradual elevation—of which process, within recent times, there is
abundant evidence on the coast-land of both islands. At both, vast
streams of more recent basaltic lavas have flowed from the interior of
the island, round and between the ancient basaltic hills; at both,
moreover, recent cones of eruption are scattered around the
circumference of the island; but at neither have eruptions taken place
within the period of history. As remarked in the last chapter, it is
probable that these ancient basaltic mountains, which resemble (at
least in many respects) the basal and disturbed remnants of two
gigantic volcanoes, owe their present form, structure, and position, to
the action of similar causes.

ST. PAUL’S ROCKS.

This small island is situated in the Atlantic Ocean, nearly one degree
north of the equator, and 540 miles distant from South America, in 29
degrees 15 minutes west longitude. Its highest point is scarcely fifty
feet above the level of the sea; its outline is irregular, and its
entire circumference barely three-quarters of a mile. This little point
of rock rises abruptly out of the ocean; and, except on its western
side, soundings were not obtained, even at the short distance of a
quarter of a mile from its shore. It is not of volcanic origin; and
this circumstance, which is the most remarkable point in its history
(as will hereafter be referred to), properly ought to exclude it from
the present volume. It is composed of rocks, unlike any which I have
met with, and which I cannot characterise by any name, and must
therefore describe.

The simplest, and one of the most abundant kinds, is a very compact,
heavy, greenish-black rock, having an angular, irregular fracture, with
some points just hard enough to scratch glass, and infusible. This
variety passes into others of paler green tints, less hard, but with a
more crystalline fracture, and translucent on their edges; and these
are fusible into a green enamel. Several other varieties are chiefly
characterised by containing innumerable threads of dark-green
serpentine, and by having calcareous matter in their interstices. These
rocks have an obscure, concretionary structure, and are full of
variously coloured angular pseudo fragments. These angular pseudo
fragments consist of the first-described dark green rock, of a brown
softer kind, of serpentine, and of a yellowish harsh stone, which,
perhaps, is related to serpentine rock. There are other vesicular,
calcareo-ferruginous, soft stones. There is no distinct stratification,
but parts are imperfectly laminated; and the whole abounds with
innumerable veins, and vein-like masses, both small and large. Of these
vein-like masses, some calcareous ones, which contain minute fragments
of shells, are clearly of subsequent origin to the others.

A GLOSSY INCRUSTATION.

Extensive portions of these rocks are coated by a layer of a glossy
polished substance, with a pearly lustre and of a greyish white colour;
it follows all the inequalities of the surface, to which it is firmly
attached. When examined with a lens, it is found to consist of numerous
exceedingly thin layers, their aggregate thickness being about the
tenth of an inch. It is considerably harder than calcareous spar, but
can be scratched with a knife; under the blowpipe it scales off,
decrepitates, slightly blackens, emits a fetid odour, and becomes
strongly alkaline: it does not effervesce in acids. (In my “Journal” I
have described this substance; I then believed that it was an impure
phosphate of lime.) I presume this substance has been deposited by
water draining from the birds’ dung, with which the rocks are covered.
At Ascension, near a cavity in the rocks which was filled with a
laminated mass of infiltrated birds’ dung, I found some irregularly
formed, stalactitical masses of apparently the same nature. These
masses, when broken, had an earthy texture; but on their outsides, and
especially at their extremities, they were formed of a pearly
substance, generally in little globules, like the enamel of teeth, but
more translucent, and so hard as just to scratch plate-glass. This
substance slightly blackens under the blowpipe, emits a bad smell, then
becomes quite white, swelling a little, and fuses into a dull white
enamel; it does not become alkaline; nor does it effervesce in acids.
The whole mass had a collapsed appearance, as if in the formation of
the hard glossy crust the whole had shrunk much. At the Abrolhos
Islands on the coast of Brazil, where also there is much birds’ dung, I
found a great quantity of a brown, arborescent substance adhering to
some trap-rock. In its arborescent form, this substance singularly
resembles some of the branched species of Nullipora. Under the
blowpipe, it behaves like the specimens from Ascension; but it is less
hard and glossy, and the surface has not the shrunk appearance.




CHAPTER III.
ASCENSION.


Basaltic lavas. Numerous craters truncated on the same side. Singular
structure of volcanic bombs. Aeriform explosions. Ejected granitic
fragments. Trachytic rocks. Singular veins. Jasper, its manner of
formation. Concretions in pumiceous tuff. Calcareous deposits and
frondescent incrustations on the coast. Remarkable laminated beds,
alternating with, and passing into, obsidian. Origin of obsidian.
Lamination of volcanic rocks.

(MAP 2: THE ISLAND OF ASCENSION.)

This island is situated in the Atlantic Ocean, in latitude 8 degrees
S., longitude 14 degrees W. It has the form of an irregular triangle
(see Map 2), each side being about six miles in length. Its highest
point is 2,870 feet (“Geographical Journal” volume 5 page 243.) above
the level of the sea. The whole is volcanic, and, from the absence of
proofs to the contrary, I believe of subaerial origin. The fundamental
rock is everywhere of a pale colour, generally compact, and of a
feldspathic nature. In the S.E. portion of the island, where the
highest land is situated, well characterised trachyte, and other
congenerous rocks of that varying family, occur. Nearly the entire
circumference is covered up by black and rugged streams of basaltic
lava, with here and there a hill or single point of rock (one of which
near the sea-coast, north of the Fort, is only two or three yards
across) of the trachyte still remaining exposed.

BASALTIC ROCKS.

The overlying basaltic lava is in some parts extremely vesicular, in
others little so; it is of a black colour, but sometimes contains
crystals of glassy feldspar, and seldom much olivine. These streams
appear to have possessed singularly little fluidity; their side walls
and lower ends being very steep, and even as much as between twenty and
thirty feet in height. Their surface is extraordinarily rugged, and
from a short distance appears as if studded with small craters. These
projections consist of broad, irregularly conical, hillocks, traversed
by fissures, and composed of the same unequally scoriaceous basalt with
the surrounding streams, but having an obscure tendency to a columnar
structure; they rise to a height between ten and thirty feet above the
general surface, and have been formed, as I presume, by the heaping up
of the viscid lava at points of greater resistance. At the base of
several of these hillocks, and occasionally likewise on more level
parts, solid ribs, composed of angulo-globular masses of basalt,
resembling in size and outline arched sewers or gutters of brickwork,
but not being hollow, project between two or three feet above the
surface of the streams; what their origin may have been, I do not know.
Many of the superficial fragments from these basaltic streams present
singularly convoluted forms; and some specimens could hardly be
distinguished from logs of dark-coloured wood without their bark.

Many of the basaltic streams can be traced, either to points of
eruption at the base of the great central mass of trachyte, or to
separate, conical, red-coloured hills, which are scattered over the
northern and western borders of the island. Standing on the central
eminence, I counted between twenty and thirty of these cones of
eruption. The greater number of them had their truncated summits cut
off obliquely, and they all sloped towards the S.E., whence the
trade-wind blows. (M. Lesson in the “Zoology of the Voyage of the
‘Coquille’” page 490 has observed this fact. Mr. Hennah (“Geolog.
Proceedings” 1835 page 189) further remarks that the most extensive
beds of ashes at Ascension invariably occur on the leeward side of the
island.) This structure no doubt has been caused by the ejected
fragments and ashes being always blown, during eruptions, in greater
quantity towards one side than towards the other. M. Moreau de Jonnes
has made a similar observation with respect to the volcanic orifices in
the West Indian Islands.

VOLCANIC BOMBS.

(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior
parts coarsely cellular, coated by a concentric layer of compact lava,
and this again by a crust of finely cellular rock.

FIGURE 4: VOLCANIC BOMB OF OBSIDIAN FROM AUSTRALIA. The upper figure
gives a front view; the lower a side view of the same object.)

These occur in great numbers strewed on the ground, and some of them
lie at considerable distances from any points of eruption. They vary in
size from that of an apple to that of a man’s body; they are either
spherical or pear-shaped, or with the hinder part (corresponding to the
tail of a comet) irregular, studded with projecting points, and even
concave. Their surfaces are rough, and fissured with branching cracks;
their internal structure is either irregularly scoriaceous and compact,
or it presents a symmetrical and very curious appearance. An irregular
segment of a bomb of this latter kind, of which I found several, is
accurately represented in Figure 3. Its size was about that of a man’s
head. The whole interior is coarsely cellular; the cells averaging in
diameter about the tenth of an inch; but nearer the outside they
gradually decrease in size. This part is succeeded by a well-defined
shell of compact lava, having a nearly uniform thickness of about the
third of an inch; and the shell is overlaid by a somewhat thicker
coating of finely cellular lava (the cells varying from the fiftieth to
the hundredth of an inch in diameter), which forms the external
surface: the line separating the shell of compact lava from the outer
scoriaceous crust is distinctly defined. This structure is very simply
explained, if we suppose a mass of viscid, scoriaceous matter, to be
projected with a rapid, rotatory motion through the air; for whilst the
external crust, from cooling, became solidified (in the state we now
see it), the centrifugal force, by relieving the pressure in the
interior parts of the bomb, would allow the heated vapours to expand
their cells; but these being driven by the same force against the
already-hardened crust, would become, the nearer they were to this
part, smaller and smaller or less expanded, until they became packed
into a solid, concentric shell. As we know that chips from a grindstone
(Nichol “Architecture of the Heavens.”) can be flirted off, when made
to revolve with sufficient velocity, we need not doubt that the
centrifugal force would have power to modify the structure of a
softened bomb, in the manner here supposed. Geologists have remarked,
that the external form of a bomb at once bespeaks the history of its
aerial course, and few now see that the internal structure can speak,
with almost equal plainness, of its rotatory movement.

M. Bory St. Vincent (“Voyage aux Quatre Isles d’Afrique” tome 1 page
222.) has described some balls of lava from the Isle of Bourbon, which
have a closely similar structure. His explanation, however (if I
understand it rightly), is very different from that which I have given;
for he supposes that they have rolled, like snowballs, down the sides
of the crater. M. Beudant (“Voyage en Hongrie” tome 2 page 214.), also,
has described some singular little balls of obsidian, never more than
six or eight inches in diameter, which he found strewed on the surface
of the ground: their form is always oval; sometimes they are much
swollen in the middle, and even spindle-shaped: their surface is
regularly marked with concentric ridges and furrows, all of which on
the same ball are at right angles to one axis: their interior is
compact and glassy. M. Beudant supposes that masses of lava, when soft,
were shot into the air, with a rotatory movement round the same axis,
and that the form and superficial ridges of the bombs were thus
produced. Sir Thomas Mitchell has given me what at first appears to be
the half of a much flattened oval ball of obsidian; it has a singular
artificial-like appearance, which is well represented (of the natural
size) in Figure 4. It was found in its present state, on a great sandy
plain between the rivers Darling and Murray, in Australia, and at the
distance of several hundred miles from any known volcanic region. It
seems to have been embedded in some reddish tufaceous matter; and may
have been transported either by the aborigines or by natural means. The
external saucer consists of compact obsidian, of a bottle-green colour,
and is filled with finely cellular black lava, much less transparent
and glassy than the obsidian. The external surface is marked with four
or five not quite perfect ridges, which are represented rather too
distinctly in Figure 4. Here, then, we have the external structure
described by M. Beudant, and the internal cellular condition of the
bombs from Ascension. The lip of the saucer is slightly concave,
exactly like the margin of a soup-plate, and its inner edge overlaps a
little the central cellular lava. This structure is so symmetrical
round the entire circumference, that one is forced to suppose that the
bomb burst during its rotatory course, before being quite solidified,
and that the lip and edges were thus slightly modified and turned
inwards. It may be remarked that the superficial ridges are in planes,
at right angles to an axis, transverse to the longer axis of the
flattened oval: to explain this circumstance, we may suppose that when
the bomb burst, the axis of rotation changed.

AERIFORM EXPLOSIONS.

The flanks of Green Mountain and the surrounding country are covered by
a great mass, some hundred feet in thickness, of loose fragments. The
lower beds generally consist of fine-grained, slightly consolidated
tuffs (Some of this peperino, or tuff, is sufficiently hard not to be
broken by the greatest force of the fingers.), and the upper beds of
great loose fragments, with alternating finer beds. (On the northern
side of the Green Mountain a thin seam, about an inch in thickness, of
compact oxide of iron, extends over a considerable area; it lies
conformably in the lower part of the stratified mass of ashes and
fragments. This substance is of a reddish- brown colour, with an almost
metallic lustre; it is not magnetic, but becomes so after having been
heated under the blowpipe, by which it is blackened and partly fused.
This seam of compact stone, by intercepting the little rain-water which
falls on the island, gives rise to a small dripping spring, first
discovered by Dampier. It is the only fresh water on the island, so
that the possibility of its being inhabited has entirely depended on
the occurrence of this ferruginous layer.) One white ribbon- like layer
of decomposed, pumiceous breccia, was curiously bent into deep unbroken
curves, beneath each of the large fragments in the superincumbent
stratum. From the relative position of these beds, I presume that a
narrow- mouthed crater, standing nearly in the position of Green
Mountain, like a great air-gun, shot forth, before its final
extinction, this vast accumulation of loose matter. Subsequently to
this event, considerable dislocations have taken place, and an oval
circus has been formed by subsidence. This sunken space lies at the
north-eastern foot of Green Mountain, and is well represented in Map 2.
Its longer axis, which is connected with a N.E. and S.W. line of
fissure, is three-fifths of a nautical mile in length; its sides are
nearly perpendicular, except in one spot, and about four hundred feet
in height; they consist, in the lower part, of a pale basalt with
feldspar, and in the upper part, of the tuff and loose ejected
fragments; the bottom is smooth and level, and under almost any other
climate a deep lake would have been formed here. From the thickness of
the bed of loose fragments, with which the surrounding country is
covered, the amount of aeriform matter necessary for their projection
must have been enormous; hence we may suppose it probable that after
the explosions vast subterranean caverns were left, and that the
falling in of the roof of one of these produced the hollow here
described. At the Galapagos Archipelago, pits of a similar character,
but of a much smaller size, frequently occur at the bases of small
cones of eruption.

EJECTED GRANITIC FRAGMENTS.

In the neighbourhood of Green Mountain, fragments of extraneous rock
are not unfrequently found embedded in the midst of masses of scoriae.
Lieutenant Evans, to whose kindness I am indebted for much information,
gave me several specimens, and I found others myself. They nearly all
have a granitic structure, are brittle, harsh to the touch, and
apparently of altered colours.

FIRST, a white syenite, streaked and mottled with red; it consists of
well- crystallised feldspar, numerous grains of quartz, and brilliant,
though small, crystals of hornblende. The feldspar and hornblende in
this and the succeeding cases have been determined by the reflecting
goniometer, and the quartz by its action under the blowpipe. The
feldspar in these ejected fragments, like the glassy kind in the
trachyte, is from its cleavage a potash-feldspar.

SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches
of a decayed mineral; one minute particle of which I was able to
ascertain, by its cleavage, to be hornblende.

THIRDLY, a mass of confusedly crystallised white feldspar, with little
nests of a dark-coloured mineral, often carious, externally rounded,
having a glossy fracture, but no distinct cleavage: from comparison
with the second specimen, I have no doubt that it is fused hornblende.

FOURTHLY, a rock, which at first appears a simple aggregation of
distinct and large-sized crystals of dusty-coloured Labrador feldspar
(Professor Miller has been so kind as to examine this mineral. He
obtained two good cleavages of 86 degrees 30 minutes and 86 degrees 50
minutes. The mean of several, which I made, was 86 degrees 30 minutes.
Professor Miller states that these crystals, when reduced to a fine
powder, are soluble in hydrochloric acid, leaving some undissolved
silex behind; the addition of oxalate of ammonia gives a copious
precipitate of lime. He further remarks, that according to Von Kobell,
anorthite (a mineral occurring in the ejected fragments at Mount Somma)
is always white and transparent, so that if this be the case, these
crystals from Ascension must be considered as Labrador feldspar.
Professor Miller adds, that he has seen an account, in Erdmann’s
“Journal fur tecnische Chemie,” of a mineral ejected from a volcano
which had the external characters of Labrador feldspar, but differed in
the analysis from that given by mineralogists of this mineral: the
author attributed this difference to an error in the analysis of
Labrador feldspar, which is very old.); but in their interstices there
is some white granular feldspar, abundant scales of mica, a little
altered hornblende, and, as I believe, no quartz. I have described
these fragments in detail, because it is rare to find granitic rocks
ejected from volcanoes with their MINERALS UNCHANGED, as is the case
with the first specimen, and partially with the second. (Daubeny, in
his work on Volcanoes page 386, remarks that this is the case; and
Humboldt, in his “Personal Narrative” volume 1 page 236, says “In
general, the masses of known primitive rocks, I mean those which
perfectly resemble our granites, gneiss, and mica-slate, are very rare
in lavas: the substances we generally denote by the name of granite,
thrown out by Vesuvius, are mixtures of nepheline, mica, and
pyroxene.”) One other large fragment, found in another spot, is
deserving of notice; it is a conglomerate, containing small fragments
of granitic, cellular, and jaspery rocks, and of hornstone porphyries,
embedded in a base of wacke, threaded by numerous thin layers of a
concretionary pitchstone passing into obsidian. These layers are
parallel, slightly tortuous, and short; they thin out at their ends,
and resemble in form the layers of quartz in gneiss. It is probable
that these small embedded fragments were not separately ejected, but
were entangled in a fluid volcanic rock, allied to obsidian; and we
shall presently see that several varieties of this latter series of
rock assume a laminated structure.

TRACHYTIC SERIES OF ROCKS.

Those occupy the more elevated and central, and likewise the
south-eastern, parts of the island. The trachyte is generally of a pale
brown colour, stained with small darker patches; it contains broken and
bent crystals of glassy feldspar, grains of specular iron, and black
microscopical points, which latter, from being easily fused, and then
becoming magnetic, I presume are hornblende. The greater number of the
hills, however, are composed of a quite white, friable stone, appearing
like a trachytic tuff. Obsidian, hornstone, and several kinds of
laminated feldspathic rocks, are associated with the trachyte. There is
no distinct stratification; nor could I distinguish a crateriform
structure in any of the hills of this series. Considerable dislocations
have taken place; and many fissures in these rocks are yet left open,
or are only partially filled with loose fragments. Within the space
(This space is nearly included by a line sweeping round Green Mountain,
and joining the hills, called the Weather Port Signal, Holyhead, and
that denominated (improperly in a geological sense) “the Crater of an
old volcano.”), mainly formed of trachyte, some basaltic streams have
burst forth; and not far from the summit of Green Mountain, there is
one stream of quite black, vesicular basalt, containing minute crystals
of glassy feldspar, which have a rounded appearance.

The soft white stone above mentioned is remarkable from its singular
resemblance, when viewed in mass, to a sedimentary tuff: it was long
before I could persuade myself that such was not its origin; and other
geologists have been perplexed by closely similar formations in
trachytic regions. In two cases, this white earthy stone formed
isolated hills; in a third, it was associated with columnar and
laminated trachyte; but I was unable to trace an actual junction. It
contains numerous crystals of glassy feldspar and black microscopical
specks, and is marked with small darker patches, exactly as in the
surrounding trachyte. Its basis, however, when viewed under the
microscope, is generally quite earthy; but sometimes it exhibits a
decidedly crystalline structure. On the hill marked “Crater of an old
volcano,” it passes into a pale greenish-grey variety, differing only
in its colour, and in not being so earthy; the passage was in one case
effected insensibly; in another, it was formed by numerous, rounded and
angular, masses of the greenish variety, being embedded in the white
variety;—in this latter case, the appearance was very much like that of
a sedimentary deposit, torn up and abraded during the deposition of a
subsequent stratum. Both these varieties are traversed by innumerable
tortuous veins (presently to be described), which are totally unlike
injected dikes, or indeed any other veins which I have ever seen. Both
varieties include a few scattered fragments, large and small, of dark-
coloured scoriaceous rocks, the cells of some of which are partially
filled with the white earthy stone; they likewise include some huge
blocks of a cellular porphyry. (The porphyry is dark coloured; it
contains numerous, often fractured, crystals of white opaque feldspar,
also decomposing crystals of oxide of iron; its vesicles include masses
of delicate, hair- like, crystals, apparently of analcime.) These
fragments project from the weathered surface, and perfectly resemble
fragments embedded in a true sedimentary tuff. But as it is known that
extraneous fragments of cellular rock are sometimes included in
columnar trachyte, in phonolite (D’Aubuisson “Traite de Geognosie” tome
2 page 548.), and in other compact lavas, this circumstance is not any
real argument for the sedimentary origin of the white earthy stone.
(Dr. Daubeny on Volcanoes, page 180 seems to have been led to believe
that certain trachytic formations of Ischia and of the Puy de Dome,
which closely resemble these of Ascension, were of sedimentary origin,
chiefly from the frequent presence in them “of scoriform portions,
different in colour from the matrix.” Dr. Daubeny adds, that on the
other hand, Brocchi, and other eminent geologists, have considered
these beds as earthy varieties of trachyte; he considers the subject
deserving of further attention.) The insensible passage of the greenish
variety into the white one, and likewise the more abrupt passage by
fragments of the former being embedded in the latter, might result from
slight differences in the composition of the same mass of molten stone,
and from the abrading action of one such part still fluid on another
part already solidified. The curiously formed veins have, I believe,
been formed by siliceous matter being subsequently segregated. But my
chief reason for believing that these soft earthy stones, with their
extraneous fragments, are not of sedimentary origin, is the extreme
improbability of crystals of feldspar, black microscopical specks, and
small stains of a darker colour occurring in the same proportional
numbers in an aqueous deposit, and in masses of solid trachyte.
Moreover, as I have remarked, the microscope occasionally reveals a
crystalline structure in the apparently earthy basis. On the other
hand, the partial decomposition of such great masses of trachyte,
forming whole mountains, is undoubtedly a circumstance of not easy
explanation.

VEINS IN THE EARTHY TRACHYTIC MASSES.

These veins are extraordinarily numerous, intersecting in the most
complicated manner both coloured varieties of the earthy trachyte: they
are best seen on the flanks of the “Crater of the old volcano.” They
contain crystals of glassy feldspar, black microscopical specks and
little dark stains, precisely as in the surrounding rock; but the basis
is very different, being exceedingly hard, compact, somewhat brittle,
and of rather less easy fusibility. The veins vary much, and suddenly,
from the tenth of an inch to one inch in thickness; they often thin
out, not only on their edges, but in their central parts, thus leaving
round, irregular apertures; their surfaces are rugged. They are
inclined at every possible angle with the horizon, or are horizontal;
they are generally curvilinear, and often interbranch one with another.
From their hardness they withstand weathering, and projecting two or
three feet above the ground, they occasionally extend some yards in
length; these plate-like veins, when struck, emit a sound, almost like
that of a drum, and they may be distinctly seen to vibrate; their
fragments, which are strewed on the ground, clatter like pieces of iron
when knocked against each other. They often assume the most singular
forms; I saw a pedestal of the earthy trachyte, covered by a
hemispherical portion of a vein, like a great umbrella, sufficiently
large to shelter two persons. I have never met with, or seen described,
any veins like these; but in form they resemble the ferruginous seams,
due to some process of segregation, occurring not uncommonly in
sandstones,—for instance, in the New Red sandstone of England. Numerous
veins of jasper and of siliceous sinter, occurring on the summit of
this same hill, show that there has been some abundant source of
silica, and as these plate-like veins differ from the trachyte only in
their greater hardness, brittleness, and less easy fusibility, it
appears probable that their origin is due to the segregation or
infiltration of siliceous matter, in the same manner as happens with
the oxides of iron in many sedimentary rocks.

SILICEOUS SINTER AND JASPER.

The siliceous sinter is either quite white, of little specific gravity,
and with a somewhat pearly fracture, passing into pinkish pearl quartz;
or it is yellowish white, with a harsh fracture, and it then contains
an earthy powder in small cavities. Both varieties occur, either in
large irregular masses in the altered trachyte, or in seams included in
broad, vertical, tortuous, irregular veins of a compact, harsh stone of
a dull red colour, appearing like a sandstone. This stone, however, is
only altered trachyte; and a nearly similar variety, but often
honeycombed, sometimes adheres to the projecting plate-like veins,
described in the last paragraph. The jasper is of an ochre yellow or
red colour; it occurs in large irregular masses, and sometimes in
veins, both in the altered trachyte and in an associated mass of
scoriaceous basalt. The cells of the scoriaceous basalt are lined or
filled with fine, concentric layers of chalcedony, coated and studded
with bright-red oxide of iron. In this rock, especially in the rather
more compact parts, irregular angular patches of the red jasper are
included, the edges of which insensibly blend into the surrounding
mass; other patches occur having an intermediate character between
perfect jasper and the ferruginous, decomposed, basaltic base. In these
patches, and likewise in the large vein-like masses of jasper, there
occur little rounded cavities, of exactly the same size and form with
the air-cells, which in the scoriaceous basalt are filled and lined
with layers of chalcedony. Small fragments of the jasper, examined
under the microscope, seem to resemble the chalcedony with its
colouring matter not separated into layers, but mingled in the
siliceous paste, together with some impurities. I can understand these
facts,—namely, the blending of the jasper into the semi-decomposed
basalt,—its occurrence in angular patches, which clearly do not occupy
pre-existing hollows in the rock,—and its containing little vesicles
filled with chalcedony, like those in the scoriaceous lava,—only on the
supposition that a fluid, probably the same fluid which deposited the
chalcedony in the air-cells, removed in those parts where there were no
cavities, the ingredients of the basaltic rock, and left in their place
silica and iron, and thus produced the jasper. In some specimens of
silicified wood, I have observed, that in the same manner as in the
basalt, the solid parts were converted into a dark-coloured homogeneous
stone, whereas the cavities formed by the larger sap-vessels (which may
be compared with the air-vesicles in the basaltic lava) and other
irregular hollows, apparently produced by decay, were filled with
concentric layers of chalcedony; in this case, there can be little
doubt that the same fluid deposited the homogeneous base and the
chalcedonic layers. After these considerations, I cannot doubt but that
the jasper of Ascension may be viewed as a volcanic rock silicified, in
precisely the same sense as this term is applied to wood, when
silicified; we are equally ignorant of the means by which every atom of
wood, whilst in a perfect state, is removed and replaced by atoms of
silica, as we are of the means by which the constituent parts of a
volcanic rock could be thus acted on. (Beudant “Voyage en Hongrie” tome
3 pages 502, 504 describes kidney-shaped masses of jasper-opal, which
either blend into the surrounding trachytic conglomerate, or are
embedded in it like chalk-flints; and he compares them with the
fragments of opalised wood, which are abundant in this same formation.
Beudant, however, appears to have viewed the process of their formation
rather as one of simple infiltration than of molecular exchange; but
the presence of a concretion, wholly different from the surrounding
matter, if not formed in a pre-existing hollow, clearly seems to me to
require, either a molecular or mechanical displacement of the atoms,
which occupied the space afterwards filled by it. The jasper-opal of
Hungary passes into chalcedony, and therefore in this case, as in that
of Ascension, jasper seems to be intimately related in origin with
chalcedony.) I was led to the careful examination of these rocks, and
to the conclusion here given, from having heard the Rev. Professor
Henslow express a similar opinion, regarding the origin in trap-rocks
of many chalcedonies and agates. Siliceous deposits seem to be very
general, if not of universal occurrence, in partially decomposed
trachytic tuffs (Beudant “Voyage Min.” tome 3 page 507 enumerates cases
in Hungary, Germany, Central France, Italy, Greece, and Mexico.); and
as these hills, according to the view above given, consist of trachyte
softened and altered in situ, the presence of free silica in this case
may be added as one more instance to the list.

CONCRETIONS IN PUMICEOUS TUFF.

The hill, marked in Map 2 “Crater of an old volcano,” has no claims to
this appellation, which I could discover, except in being surmounted by
a circular, very shallow, saucer-like summit, nearly half a mile in
diameter. This hollow has been nearly filled up with many successive
sheets of ashes and scoriae, of different colours, and slightly
consolidated. Each successive saucer-shaped layer crops out all round
the margin, forming so many rings of various colours, and giving to the
hill a fantastic appearance. The outer ring is broad, and of a white
colour; hence it resembles a course round which horses have been
exercised, and has received the name of the Devil’s Riding School, by
which it is most generally known. These successive layers of ashes must
have fallen over the whole surrounding country, but they have all been
blown away except in this one hollow, in which probably moisture
accumulated, either during an extraordinary year when rain fell, or
during the storms often accompanying volcanic eruptions. One of the
layers of a pinkish colour, and chiefly derived from small, decomposed
fragments of pumice, is remarkable, from containing numerous
concretions. These are generally spherical, from half an inch to three
inches in diameter; but they are occasionally cylindrical, like those
of iron-pyrites in the chalk of Europe. They consist of a very tough,
compact, pale-brown stone, with a smooth and even fracture. They are
divided into concentric layers by thin white partitions, resembling the
external superficies; six or eight of such layers are distinctly
defined near the outside; but those towards the inside generally become
indistinct, and blend into a homogeneous mass. I presume that these
concentric layers were formed by the shrinking of the concretion, as it
became compact. The interior part is generally fissured by minute
cracks or septaria, which are lined, both by black, metallic, and by
other white and crystalline specks, the nature of which I was unable to
ascertain. Some of the larger concretions consist of a mere spherical
shell, filled with slightly consolidated ashes. The concretions contain
a small proportion of carbonate of lime: a fragment placed under the
blowpipe decrepitates, then whitens and fuses into a blebby enamel, but
does not become caustic. The surrounding ashes do not contain any
carbonate of lime; hence the concretions have probably been formed, as
is so often the case, by the aggregation of this substance. I have not
met with any account of similar concretions; and considering their
great toughness and compactness, their occurrence in a bed, which
probably has been subjected only to atmospheric moisture, is
remarkable.

FORMATION OF CALCAREOUS ROCKS ON THE SEA-COAST.

On several of the sea-beaches, there are immense accumulations of
small, well-rounded particles of shells and corals, of white,
yellowish, and pink colours, interspersed with a few volcanic
particles. At the depth of a few feet, these are found cemented
together into stone, of which the softer varieties are used for
building; there are other varieties, both coarse and fine-grained, too
hard for this purpose: and I saw one mass divided into even layers half
an inch in thickness, which were so compact that when struck with a
hammer they rang like flint. It is believed by the inhabitants, that
the particles become united in the course of a single year. The union
is effected by calcareous matter; and in the most compact varieties,
each rounded particle of shell and volcanic rock can be distinctly seen
to be enveloped in a husk of pellucid carbonate of lime. Extremely few
perfect shells are embedded in these agglutinated masses; and I have
examined even a large fragment under a microscope, without being able
to discover the least vestige of striae or other marks of external
form: this shows how long each particle must have been rolled about,
before its turn came to be embedded and cemented. (The eggs of the
turtle being buried by the parent, sometimes become enclosed in the
solid rock. Mr. Lyell has given a figure (“Principles of Geology” book
3 chapter 17) of some eggs, containing the bones of young turtles,
found thus entombed.) One of the most compact varieties, when placed in
acid, was entirely dissolved, with the exception of some flocculent
animal matter; its specific gravity was 2.63. The specific gravity of
ordinary limestone varies from 2.6 to 2.75; pure Carrara marble was
found by Sir H. De la Beche to be 2.7. (“Researches in Theoretical
Geology” page 12.) It is remarkable that these rocks of Ascension,
formed close to the surface, should be nearly as compact as marble,
which has undergone the action of heat and pressure in the plutonic
regions.

The great accumulation of loose calcareous particles, lying on the
beach near the Settlement, commences in the month of October, moving
towards the S.W., which, as I was informed by Lieutenant Evans, is
caused by a change in the prevailing direction of the currents. At this
period the tidal rocks, at the S.W. end of the beach, where the
calcareous sand is accumulating, and round which the currents sweep,
become gradually coated with a calcareous incrustation, half an inch in
thickness. It is quite white, compact, with some parts slightly
spathose, and is firmly attached to the rock. After a short time it
gradually disappears, being either redissolved, when the water is less
charged with lime, or more probably is mechanically abraded. Lieutenant
Evans has observed these facts, during the six years he has resided at
Ascension. The incrustation varies in thickness in different years: in
1831 it was unusually thick. When I was there in July, there was no
remnant of the incrustation; but on a point of basalt, from which the
quarrymen had lately removed a mass of the calcareous freestone, the
incrustation was perfectly preserved. Considering the position of the
tidal-rocks, and the period at which they become coated, there can be
no doubt that the movement and disturbance of the vast accumulation of
calcareous particles, many of them being partially agglutinated
together, cause the waves of the sea to be so highly charged with
carbonate of lime, that they deposit it on the first objects against
which they impinge. I have been informed by Lieutenant Holland, R.N.,
that this incrustation is formed on many parts of the coast, on most of
which, I believe, there are likewise great masses of comminuted shells.

A FRONDESCENT CALCAREOUS INCRUSTATION.

(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the
tidal-rocks at Ascension.)

In many respects this is a singular deposit; it coats throughout the
year the tidal volcanic rocks, that project from the beaches composed
of broken shells. Its general appearance is well represented in Figure
5; but the fronds or discs, of which it is composed, are generally so
closely crowded together as to touch. These fronds have their sinuous
edges finely crenulated, and they project over their pedestals or
supports; their upper surfaces are either slightly concave, or slightly
convex; they are highly polished, and of a dark grey or jet black
colour; their form is irregular, generally circular, and from the tenth
of an inch to one inch and a half in diameter; their thickness, or
amount of their projection from the rock on which they stand, varies
much, about a quarter of an inch being perhaps most usual. The fronds
occasionally become more and more convex, until they pass into
botryoidal masses with their summits fissured; when in this state, they
are glossy and of an intense black, so as to resemble some fused
metallic substance. I have shown the incrustation, both in this latter
and in its ordinary state to several geologists, but not one could
conjecture its origin, except that perhaps it was of volcanic nature!

The substance forming the fronds has a very compact and often almost
crystalline fracture; the edges being translucent, and hard enough
easily to scratch calcareous spar. Under the blowpipe it immediately
becomes white, and emits a strong animal odour, like that from fresh
shells. It is chiefly composed of carbonate of lime; when placed in
muriatic acid it froths much, leaving a residue of sulphate of lime,
and of an oxide of iron, together with a black powder, which is not
soluble in heated acids. This latter substance seems to be
carbonaceous, and is evidently the colouring matter. The sulphate of
lime is extraneous, and occurs in distinct, excessively minute,
lamellar plates, studded on the surface of the fronds, and embedded
between the fine layers of which they are composed; when a fragment is
heated in the blowpipe, these lamellae are immediately rendered
visible. The original outline of the fronds may often be traced, either
to a minute particle of shell fixed in a crevice of the rock, or to
several cemented together; these first become deeply corroded, by the
dissolving power of the waves, into sharp ridges, and then are coated
with successive layers of the glossy, grey, calcareous incrustation.
The inequalities of the primary support affect the outline of every
successive layer, in the same manner as may often be seen in
bezoar-stones, when an object like a nail forms the centre of
aggregation. The crenulated edges, however, of the frond appear to be
due to the corroding power of the surf on its own deposit, alternating
with fresh depositions. On some smooth basaltic rocks on the coast of
St. Jago, I found an exceedingly thin layer of brown calcareous matter,
which under a lens presented a miniature likeness of the crenulated and
polished fronds of Ascension; in this case a basis was not afforded by
any projecting extraneous particles. Although the incrustation at
Ascension is persistent throughout the year; yet from the abraded
appearance of some parts, and from the fresh appearance of other parts,
the whole seems to undergo a round of decay and renovation, due
probably to changes in the form of the shifting beach, and consequently
in the action of the breakers: hence probably it is, that the
incrustation never acquires a great thickness. Considering the position
of the encrusted rocks in the midst of the calcareous beach, together
with its composition, I think there can be no doubt that its origin is
due to the dissolution and subsequent deposition of the matter
composing the rounded particles of shells and corals. (The selenite, as
I have remarked is extraneous, and must have been derived from the
sea-water. It is an interesting circumstance thus to find the waves of
the ocean, sufficiently charged with sulphate of lime, to deposit it on
the rocks, against which they dash every tide. Dr. Webster has
described (“Voyage of the ‘Chanticleer’” volume 2 page 319) beds of
gypsum and salt, as much as two feet in thickness, left by the
evaporation of the spray on the rocks on the windward coast. Beautiful
stalactites of selenite, resembling in form those of carbonate of lime,
are formed near these beds. Amorphous masses of gypsum, also, occur in
caverns in the interior of the island; and at Cross Hill (an old
crater) I saw a considerable quantity of salt oozing from a pile of
scoriae. In these latter cases, the salt and gypsum appear to be
volcanic products.) From this source it derives its animal matter,
which is evidently the colouring principle. The nature of the deposit,
in its incipient stage, can often be well seen upon a fragment of white
shell, when jammed between two of the fronds; it then appears exactly
like the thinnest wash of a pale grey varnish. Its darkness varies a
little, but the jet blackness of some of the fronds and of the
botryoidal masses seems due to the translucency of the successive grey
layers. There is, however, this singular circumstance, that when
deposited on the under side of ledges of rock or in fissures, it
appears always to be of a pale, pearly grey colour, even when of
considerable thickness: hence one is led to suppose, that an abundance
of light is necessary to the development of the dark colour, in the
same manner as seems to be the case with the upper and exposed surfaces
of the shells of living mollusca, which are always dark, compared with
their under surfaces and with the parts habitually covered by the
mantle of the animal. In this circumstance,—in the immediate loss of
colour and in the odour emitted under the blowpipe,—in the degree of
hardness and translucency of the edges,—and in the beautiful polish of
the surface (From the fact described in my “Journal of Researches” of a
coating of oxide of iron, deposited by a streamlet on the rocks in its
bed (like a nearly similar coating at the great cataracts of the
Orinoco and Nile), becoming finely polished where the surf acts, I
presume that the surf in this instance, also, is the polishing agent.),
rivalling when in a fresh state that of the finest Oliva, there is a
striking analogy between this inorganic incrustation and the shells of
living molluscous animals. (In the section descriptive of St. Paul’s
Rocks, I have described a glossy, pearly substance, which coats the
rocks, and an allied stalactitical incrustation from Ascension, the
crust of which resembles the enamel of teeth, but is hard enough to
scratch plate-glass. Both these substances contain animal matter, and
seem to have been derived from water in filtering through birds’ dung.)
This appears to me to be an interesting physiological fact. (Mr. Horner
and Sir David Brewster have described “Philosophical Transactions” 1836
page 65 a singular “artificial substance, resembling shell.” It is
deposited in fine, transparent, highly polished, brown- coloured
laminae, possessing peculiar optical properties, on the inside of a
vessel, in which cloth, first prepared with glue and then with lime, is
made to revolve rapidly in water. It is much softer, more transparent,
and contains more animal matter, than the natural incrustation at
Ascension; but we here again see the strong tendency which carbonate of
lime and animal matter evince to form a solid substance allied to
shell.)

SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN.

These beds occur within the trachytic district, at the western base of
Green Mountain, under which they dip at a high inclination. They are
only partially exposed, being covered up by modern ejections; from this
cause, I was unable to trace their junction with the trachyte, or to
discover whether they had flowed as a stream of lava, or had been
injected amidst the overlying strata. There are three principal beds of
obsidian, of which the thickest forms the base of the section. The
alternating stony layers appear to me eminently curious, and shall be
first described, and afterwards their passage into the obsidian. They
have an extremely diversified appearance; five principal varieties may
be noticed, but these insensibly blend into each other by endless
gradations.

FIRST.

A pale grey, irregularly and coarsely laminated (This term is open to
some misinterpretation, as it may be applied both to rocks divided into
laminae of exactly the same composition, and to layers firmly attached
to each other, with no fissile tendency, but composed of different
minerals, or of different shades of colour. The term “laminated,” in
this chapter, is applied in these latter senses; where a homogeneous
rock splits, as in the former sense, in a given direction, like
clay-slate, I have used the term “fissile.”), harsh-feeling rock,
resembling clay-slate which has been in contact with a trap-dike, and
with a fracture of about the same degree of crystalline structure. This
rock, as well as the following varieties, easily fuses into a pale
glass. The greater part is honeycombed with irregular, angular,
cavities, so that the whole has a curious appearance, and some
fragments resemble in a remarkable manner silicified logs of decayed
wood. This variety, especially where more compact, is often marked with
thin whitish streaks, which are either straight or wrap round, one
behind the other, the elongated carious hollows.

SECONDLY.

A bluish grey or pale brown, compact, heavy, homogeneous stone, with an
angular, uneven, earthy fracture; viewed, however, under a lens of high
power, the fracture is seen to be distinctly crystalline, and even
separate minerals can be distinguished.

THIRDLY.

A stone of the same kind with the last, but streaked with numerous,
parallel, slightly tortuous, white lines of the thickness of hairs.
These white lines are more crystalline than the parts between them; and
the stone splits along them: they frequently expand into exceedingly
thin cavities, which are often only just perceptible with a lens. The
matter forming the white lines becomes better crystallised in these
cavities, and Professor Miller was fortunate enough, after several
trials, to ascertain that the white crystals, which are the largest,
were of quartz (Professor Miller informs me that the crystals which he
measured had the faces P, z, m of the figure (147) given by Haidinger
in his Translation of Mohs; and he adds, that it is remarkable, that
none of them had the slightest trace of faces r of the regular
six-sided prism.), and that the minute green transparent needles were
augite, or, as they would more generally be called, diopside: besides
these crystals, there are some minute, dark specks without a trace of
crystalline, and some fine, white, granular, crystalline matter which
is probably feldspar. Minute fragments of this rock are easily fusible.

FOURTHLY.

A compact crystalline rock, banded in straight lines with innumerable
layers of white and grey shades of colour, varying in width from the
thirtieth to the two-hundredth of an inch; these layers seem to be
composed chiefly of feldspar, and they contain numerous perfect
crystals of glassy feldspar, which are placed lengthways; they are also
thickly studded with microscopically minute, amorphous, black specks,
which are placed in rows, either standing separately, or more
frequently united, two or three or several together, into black lines,
thinner than a hair. When a small fragment is heated in the blowpipe,
the black specks are easily fused into black brilliant beads, which
become magnetic,—characters that apply to no common mineral except
hornblende or augite. With the black specks there are mingled some
others of a red colour, which are magnetic before being heated, and no
doubt are oxide of iron. Round two little cavities, in a specimen of
this variety, I found the black specks aggregated into minute crystals,
appearing like those of augite or hornblende, but too dull and small to
be measured by the goniometer; in the specimen, also, I could
distinguish amidst the crystalline feldspar, grains, which had the
aspect of quartz. By trying with a parallel ruler, I found that the
thin grey layers and the black hair-like lines were absolutely straight
and parallel to each other. It is impossible to trace the gradation
from the homogeneous grey rocks to these striped varieties, or indeed
the character of the different layers in the same specimen, without
feeling convinced that the more or less perfect whiteness of the
crystalline feldspathic matter depends on the more or less perfect
aggregation of diffused matter, into the black and red specks of
hornblende and oxide of iron.

FIFTHLY.

A compact heavy rock, not laminated, with an irregular, angular, highly
crystalline, fracture; it abounds with distinct crystals of glassy
feldspar, and the crystalline feldspathic base is mottled with a black
mineral, which on the weathered surface is seen to be aggregated into
small crystals, some perfect, but the greater number imperfect. I
showed this specimen to an experienced geologist, and asked him what it
was; he answered, as I think every one else would have done, that it
was a primitive greenstone. The weathered surface, also, of the banded
variety in Figure 4, strikingly resembles a worn fragment of finely
laminated gneiss.

These five varieties, with many intermediate ones, pass and repass into
each other. As the compact varieties are quite subordinate to the
others, the whole may be considered as laminated or striped. The
laminae, to sum up their characteristics, are either quite straight, or
slightly tortuous, or convoluted; they are all parallel to each other,
and to the intercalating strata of obsidian; they are generally of
extreme thinness; they consist either of an apparently homogeneous,
compact rock, striped with different shades of grey and brown colours,
or of crystalline feldspathic layers in a more or less perfect state of
purity, and of different thicknesses, with distinct crystals of glassy
feldspar placed lengthways, or of very thin layers chiefly composed of
minute crystals of quartz and augite, or composed of black and red
specks of an augitic mineral and of an oxide of iron, either not
crystallised or imperfectly so. After having fully described the
obsidian, I shall return to the subject of the lamination of rocks of
the trachytic series.

The passage of the foregoing beds into the strata of glassy obsidian is
effected in several ways: first, angulo-modular masses of obsidian,
both large and small, abruptly appear disseminated in a slaty, or in an
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
fracture. Secondly, small irregular nodules of the obsidian, either
standing separately, or united into thin layers, seldom more than the
tenth of an inch in thickness, alternate repeatedly with very thin
layers of a feldspathic rock, which is striped with the finest parallel
zones of colour, like an agate, and which sometimes passes into the
nature of pitchstone; the interstices between the nodules of obsidian
are generally filled by soft white matter, resembling pumiceous ashes.
Thirdly, the whole substance of the bounding rock suddenly passes into
an angulo-concretionary mass of obsidian. Such masses (as well as the
small nodules) of obsidian are of a pale green colour, and are
generally streaked with different shades of colour, parallel to the
laminae of the surrounding rock; they likewise generally contain minute
white sphaerulites, of which half is sometimes embedded in a zone of
one shade of colour, and half in a zone of another shade. The obsidian
assumes its jet black colour and perfectly conchoidal fracture, only
when in large masses; but even in these, on careful examination and on
holding the specimens in different lights, I could generally
distinguish parallel streaks of different shades of darkness.

(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The
upper ones are externally marked with parallel ridges. The internal
radiating structure of the lower ones, is much too plainly represented.

FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES,
INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly
the natural size.)

One of the commonest transitional rocks deserves in several respects a
further description. It is of a very complicated nature, and consists
of numerous thin, slightly tortuous layers of a pale-coloured
feldspathic stone, often passing into an imperfect pitchstone,
alternating with layers formed of numberless little globules of two
varieties of obsidian, and of two kinds of sphaerulites, embedded in a
soft or in a hard pearly base. The sphaerulites are either white and
translucent, or dark brown and opaque; the former are quite spherical,
of small size, and distinctly radiated from their centre. The dark
brown sphaerulites are less perfectly round, and vary in diameter from
the twentieth to the thirtieth of an inch; when broken they exhibit
towards their centres, which are whitish, an obscure radiating
structure; two of them when united sometimes have only one central
point of radiation; there is occasionally a trace of or a hollow
crevice in their centres. They stand either separately, or are united
two or three or many together into irregular groups, or more commonly
into layers, parallel to the stratification of the mass. This union in
many cases is so perfect, that the two sides of the layer thus formed,
are quite even; and these layers, as they become less brown and opaque,
cannot be distinguished from the alternating layers of the
pale-coloured feldspathic stone. The sphaerulites, when not united, are
generally compressed in the plane of the lamination of the mass; and in
this same plane, they are often marked internally, by zones of
different shades of colour, and externally by small ridges and furrows.
In the upper part of Figure 6, the sphaerulites with the parallel
ridges and furrows are represented on an enlarged scale, but they are
not well executed; and in the lower part, their usual manner of
grouping is shown. In another specimen, a thin layer formed of the
brown sphaerulites closely united together, intersects, as represented
in Figure 7, a layer of similar composition; and after running for a
short space in a slightly curved line, again intersects it, and
likewise a second layer lying a little way beneath that first
intersected. The small nodules also of obsidian are sometimes
externally marked with ridges and furrows, parallel to the lamination
of the mass, but always less plainly than the sphaerulites. These
obsidian nodules are generally angular, with their edges blunted: they
are often impressed with the form of the adjoining sphaerulites, than
which they are always larger; the separate nodules seldom appear to
have drawn each other out by exerting a mutually attractive force. Had
I not found in some cases, a distinct centre of attraction in these
nodules of obsidian, I should have been led to have considered them as
residuary matter, left during the formation of the pearlstone, in which
they are embedded, and of the sphaerulitic globules.

The sphaerulites and the little nodules of obsidian in these rocks so
closely resemble, in general form and structure, concretions in
sedimentary deposits, that one is at once tempted to attribute to them
an analogous origin. They resemble ordinary concretions in the
following respects: in their external form,—in the union of two or
three, or of several, into an irregular mass, or into an even-sided
layer,—in the occasional intersection of one such layer by another, as
in the case of chalk-flints,- -in the presence of two or three kinds of
nodules, often close together, in the same basis,—in their fibrous,
radiating structure, with occasional hollows in their centres,—in the
co-existence of a laminary, concretionary, and radiating structure, as
is so well developed in the concretions of magnesian limestone,
described by Professor Sedgwick. (“Geological Transactions” volume 3
part 1 page 37.) Concretions in sedimentary deposits, it is known, are
due to the separation from the surrounding mass of the whole or part of
some mineral substance, and its aggregation round certain points of
attraction. Guided by this fact, I have endeavoured to discover whether
obsidian and the sphaerulites (to which may be added marekanite and
pearlstone, both of them occurring in nodular concretions in the
trachytic series) differ in their constituent parts, from the minerals
generally composing trachytic rocks. It appears from three analyses,
that obsidian contains on an average 76 per cent of silica; from one
analysis, that sphaerulites contain 79.12; from two, that marekanite
contains 79.25; and from two other analyses, that pearlstone contains
75.62 of silica. (The foregoing analyses are taken from Beudant “Traite
de Mineralogie” tome 2 page 113; and one analysis of obsidian from
Phillips “Mineralogy.”) Now, the constituent parts of trachyte, as far
as they can be distinguished consist of feldspar, containing 65.21 of
silica; or of albite, containing 69.09; of hornblende, containing 55.27
(These analyses are taken from Von Kobell “Grundzuge der Mineralogie”
1838.), and of oxide of iron: so that the foregoing glassy
concretionary substances all contain a larger proportion of silica than
that occurring in ordinary feldspathic or trachytic rocks. D’Aubuisson
(“Traite de Geogn.” tome 2 page 535.), also, has remarked on the large
proportion of silica compared with alumina, in six analyses of obsidian
and pearlstone given in Brongniart’s “Mineralogy.” Hence I conclude,
that the foregoing concretions have been formed by a process of
aggregation, strictly analogous to that which takes place in aqueous
deposits, acting chiefly on the silica, but likewise on some of the
other elements of the surrounding mass, and thus producing the
different concretionary varieties. From the well-known effects of rapid
cooling (This is seen in the manufacture of common glass, and in
Gregory Watts’s experiments on molten trap; also on the natural
surfaces of lava- streams, and on the side-walls of dikes.) in giving
glassiness of texture, it is probably necessary that the entire mass,
in cases like that of Ascension, should have cooled at a certain rate;
but considering the repeated and complicated alterations of nodules and
thin layers of a glassy texture with other layers quite stony or
crystalline, all within the space of a few feet or even inches, it is
hardly possible that they could have cooled at different rates, and
thus have acquired their different textures.

The natural sphaerulites in these rocks very closely resemble those
produced in glass, when slowly cooled. (I do not know whether it is
generally known, that bodies having exactly the same appearance as
sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in
an agate, formed within a cavity in a piece of silicified wood, some
little specks, which were only just visible to the naked eye: these
specks, when placed by him under a lens of high power, presented a
beautiful appearance: they were perfectly circular, and consisted of
the finest fibres of a brown colour, radiating with great exactness
from a common centre. These little radiating stars are occasionally
intersected, and portions are quite cut off by the fine, ribbon-like
zones of colour in the agate. In the obsidian of Ascension, the halves
of a sphaerulite often lie in different zones of colour, but they are
not cut off by them, as in the agate.) In some fine specimens of
partially devitrified glass, in the possession of Mr. Stokes, the
sphaerulites are united into straight layers with even sides, parallel
to each other, and to one of the outer surfaces, exactly as in the
obsidian. These layers sometimes interbranch and form loops; but I did
not see any case of actual intersection. They form the passage from the
perfectly glassy portions, to those nearly homogeneous and stony, with
only an obscure concretionary structure. In the same specimen, also,
sphaerulites differing slightly in colour and in structure, occur
embedded close together. Considering these facts, it is some
confirmation of the view above given of the concretionary origin of the
obsidian and natural sphaerulites, to find that M. Dartigues (“Journal
de Physique” tome 59 1804 pages 10, 12.), in his curious paper on this
subject, attributes the production of sphaerulites in glass, to the
different ingredients obeying their own laws of attraction and becoming
aggregated. He is led to believe that this takes place, from the
difficulty in remelting sphaerulitic glass, without the whole be first
thoroughly pounded and mixed together; and likewise from the fact, that
the change takes place most readily in glass composed of many
ingredients. In confirmation of M. Dartigues’ view, I may remark, that
M. Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found that the
sphaerulitic portions of devitrified glass were acted on both by nitric
acid and under the blowpipe, in a different manner from the compact
paste in which they were embedded.

COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION,
WITH THOSE OF OTHER COUNTRIES.

I have been struck with much surprise, how closely the excellent
description of the obsidian rocks of Hungary, given by Beudant (“Voyage
en Hongrie” tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages
369, 371, 377, 381.), and that by Humboldt, of the same formation in
Mexico and Peru (“Essai Geognostique” pages 176, 326, 328.), and
likewise the descriptions given by several authors (P. Scrope
“Geological Transactions” volume 2 second series page 195. Consult also
Dolomieu “Voyage aux Isles Lipari” and D’Aubuisson “Traite de Geogn.”
tome 2 page 534.) of the trachytic regions in the Italian islands,
agree with my observations at Ascension. Many passages might have been
transferred without alteration from the works of the above authors, and
would have been applicable to this island. They all agree in the
laminated and stratified character of the whole series; and Humboldt
speaks of some of the beds of obsidian being ribboned like jasper. (In
Mr. Stokes’ fine collection of obsidians from Mexico, I observe that
the sphaerulites are generally much larger than those of Ascension;
they are generally white, opaque, and are united into distinct layers:
there are many singular varieties, different from any at Ascension. The
obsidians are finely zoned, in quite straight or curved lines, with
exceedingly slight differences of tint, of cellularity, and of more or
less perfect degrees of glassiness. Tracing some of the less perfectly
glassy zones, they are seen to become studded with minute white
sphaerulites, which become more and more numerous, until at last they
unite and form a distinct layer: on the other hand, at Ascension, only
the brown sphaerulites unite and form layers; the white ones always
being irregularly disseminated. Some specimens at the Geological
Society, said to belong to an obsidian formation from Mexico, have an
earthy fracture, and are divided in the finest parallel laminae, by
specks of a black mineral, like the augitic or hornblendic specks in
the rocks at Ascension.) They all agree in the nodular or concretionary
character of the obsidian, and of the passage of these nodules into
layers. They all refer to the repeated alterations, often in undulatory
planes, of glassy, pearly, stony, and crystalline layers: the
crystalline layers, however, seem to be much more perfectly developed
at Ascension, than in the above-named countries. Humboldt compares some
of the stony beds, when viewed from a distance, to strata of a
schistose sandstone. Sphaerulites are described as occurring abundantly
in all cases; and they everywhere seem to mark the passage, from the
perfectly glassy to the stony and crystalline beds. Beudant’s account
(Beudant “Voyage” tome 3 page 373.) of his “perlite lithoide
globulaire” in every, even the most trifling particular, might have
been written for the little brown sphaerulitic globules of the rocks of
Ascension.

From the close similarity in so many respects, between the obsidian
formations of Hungary, Mexico, Peru, and of some of the Italian
islands, with that of Ascension, I can hardly doubt that in all these
cases, the obsidian and the sphaerulites owe their origin to a
concretionary aggregation of the silica, and of some of the other
constituent elements, taking place whilst the liquified mass cooled at
a certain required rate. It is, however, well-known, that in several
places, obsidian has flowed in streams like lava; for instance, at
Teneriffe, at the Lipari Islands, and at Iceland. (For Teneriffe see
von Buch “Descript. des Isles Canaries” pages 184 and 190; for the
Lipari Islands see Dolomieu “Voyage” page 34; for Iceland see Mackenzie
“Travels” page 369.) In these cases, the superficial parts are the most
perfectly glassy, the obsidian passing at the depth of a few feet into
an opaque stone. In an analysis by Vauquelin of a specimen of obsidian
from Hecla, which probably flowed as lava, the proportion of silica is
nearly the same as in the nodular or concretionary obsidian from
Mexico. It would be interesting to ascertain, whether the opaque
interior portions and the superficial glassy coating contained the same
proportional constituent parts: we know from M. Dufrenoy (“Memoires
pour servir a une Descript. Geolog. de la France” tome 4 page 371.)
that the exterior and interior parts of the same stream of lava
sometimes differ considerably in their composition. Even should the
whole body of the stream of obsidian turn out to be similarly composed
with nodular obsidian, it would only be necessary, in accordance with
the foregoing facts, to suppose that lava in these instances had been
erupted with its ingredients mixed in the same proportion, as in the
concretionary obsidian.

LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES.

We have seen that, in several and widely distant countries, the strata
alternating with beds of obsidian, are highly laminated. The nodules,
also, both large and small, of the obsidian, are zoned with different
shades of colour; and I have seen a specimen from Mexico in Mr. Stokes’
collection, with its external surface weathered (MacCulloch states
“Classification of Rocks” page 531 that the exposed surfaces of the
pitchstone dikes in Arran are furrowed “with undulating lines,
resembling certain varieties of marbled paper, and which evidently
result from some corresponding difference of laminar structure.”) into
ridges and furrows, corresponding with the zones of different degrees
of glassiness: Humboldt (“Personal Narrative” volume 1 page 222.),
moreover, found on the Peak of Teneriffe, a stream of obsidian divided
by very thin, alternating, layers of pumice. Many other lavas of the
feldspathic series are laminated; thus, masses of common trachyte at
Ascension are divided by fine earthy lines, along which the rock
splits, separating thin layers of slightly different shades of colour;
the greater number, also, of the embedded crystals of glassy feldspar
are placed lengthways in the same direction. Mr. P. Scrope (“Geological
Transactions” volume 2 second series page 195.) has described a
remarkable columnar trachyte in the Panza Islands, which seems to have
been injected into an overlying mass of trachytic conglomerate: it is
striped with zones, often of extreme tenuity, of different textures and
colours; the harder and darker zones appearing to contain a larger
proportion of silica. In another part of the island, there are layers
of pearlstone and pitchstone, which in many respects resemble those of
Ascension. The zones in the columnar trachyte are generally contorted;
they extend uninterruptedly for a great length in a vertical direction,
and apparently parallel to the walls of the dike-like mass. Von Buch
(“Description des Iles Canaries” page 184.) has described at Teneriffe,
a stream of lava containing innumerable thin, plate-like crystals of
feldspar, which are arranged like white threads, one behind the other,
and which mostly follow the same direction. Dolomieu (“Voyage aux Isles
de Lipari” pages 35 and 85.) also states, that the grey lavas of the
modern cone of Vulcano, which have a vitreous texture, are streaked
with parallel white lines: he further describes a solid pumice-stone
which possesses a fissile structure, like that of certain micaceous
schists. Phonolite, which I may observe is often, if not always, an
injected rock, also, often has a fissile structure; this is generally
due to the parallel position of the embedded crystals of feldspar, but
sometimes, as at Fernando Noronha, seems to be nearly independent of
their presence. (In this case, and in that of the fissile pumice-stone,
the structure is very different from that in the foregoing cases, where
the laminae consist of alternate layers of different composition or
texture. In some sedimentary formations, however, which apparently are
homogeneous and fissile, as in glossy clay-slate, there is reason to
believe, according to D’Aubuisson, that the laminae are really due to
excessively thin, alternating, layers of mica.) From these facts we
see, that various rocks of the feldspathic series have either a
laminated or fissile structure, and that it occurs both in masses which
have injected into overlying strata, and in others which have flowed as
streams of lava.

The laminae of the beds, alternating with the obsidian at Ascension,
dip at a high angle under the mountain, at the base of which they are
situated; and they do not appear as if they had been inclined by
violence. A high inclination is common to these beds in Mexico, Peru,
and in some of the Italian islands (See Phillips “Mineralogy” for the
Italian Islands page 136. For Mexico and Peru see Humboldt “Essai
Geognostique.” Mr. Edwards also describes the high inclination of the
obsidian rocks of the Cerro del Navaja in Mexico in the “Proc. of the
Geolog. Soc.” June 1838.): on the other hand, in Hungary, the layers
are horizontal; the laminae, also, of some of the lava-streams above
referred to, as far as I can understand the descriptions given of them,
appear to be highly inclined or vertical. I doubt whether in any of
these cases, the laminae have been tilted into their present position;
and in some instances, as in that of the trachyte described by Mr.
Scrope, it is almost certain that they have been originally formed with
a high inclination. In many of these cases, there is evidence that the
mass of liquified rock has moved in the direction of the laminae. At
Ascension, many of the air-cells have a drawn out appearance, and are
crossed by coarse semi-glassy fibres, in the direction of the laminae;
and some of the layers, separating the sphaerulitic globules, have a
scored appearance, as if produced by the grating of the globules. I
have seen a specimen of zoned obsidian from Mexico, in Mr. Stokes’
collection, with the surfaces of the best-defined layers streaked or
furrowed with parallel lines; and these lines or streaks precisely
resembled those, produced on the surface of a mass of artificial glass
by its having been poured out of a vessel. Humboldt, also, has
described little cavities, which he compares to the tails of comets,
behind sphaerulites in laminated obsidian rocks from Mexico, and Mr.
Scrope has described other cavities behind fragments embedded in his
laminated trachyte, and which he supposes to have been produced during
the movement of the mass. (“Geological Transactions” volume 2 second
series page 200 etc. These embedded fragments, in some instances,
consist of the laminated trachyte broken off and “enveloped in those
parts, which still remained liquid.” Beudant, also, frequently refers
in his great work on “Hungary” tome 3 page 386, to trachytic rocks,
irregularly spotted with fragments of the same varieties, which in
other parts form the parallel ribbons. In these cases, we must suppose,
that after part of the molten mass had assumed a laminated structure, a
fresh irruption of lava broke up the mass, and involved fragments, and
that subsequently the whole became relaminated.) From such facts, most
authors have attributed the lamination of these volcanic rocks to their
movement whilst liquified. Although it is easy to perceive, why each
separate air-cell, or each fibre in pumice-stone (Dolomieu “Voyage”
page 64.), should be drawn out in the direction of the moving mass; it
is by no means at first obvious why such air-cells and fibres should be
arranged by the movement, in the same planes, in laminae absolutely
straight and parallel to each other, and often of extreme tenuity; and
still less obvious is it, why such layers should come to be of slightly
different composition and of different textures.

In endeavouring to make out the cause of the lamination of these
igneous feldspathic rocks, let us return to the facts so minutely
described at Ascension. We there see, that some of the thinnest layers
are chiefly formed by numerous, exceedingly minute, though perfect,
crystals of different minerals; that other layers are formed by the
union of different kinds of concretionary globules, and that the layers
thus formed, often cannot be distinguished from the ordinary
feldspathic and pitchstone layers, composing a large portion of the
entire mass. The fibrous radiating structure of the sphaerulites seems,
judging from many analogous cases, to connect the concretionary and
crystalline forces: the separate crystals, also, of feldspar all lie in
the same parallel planes. (The formation, indeed, of a large crystal of
any mineral in a rock of mixed composition implies an aggregation of
the requisite atoms, allied to concretionary action. The cause of the
crystals of feldspar in these rocks of Ascension, being all placed
lengthways, is probably the same with that which elongates and flattens
all the brown sphaerulitic globules (which behave like feldspar under
the blowpipe) in this same direction.) These allied forces, therefore,
have played an important part in the lamination of the mass, but they
cannot be considered the primary force; for the several kinds of
nodules, both the smallest and largest, are internally zoned with
excessively fine shades of colour, parallel to the lamination of the
whole; and many of them are, also, externally marked in the same
direction with parallel ridges and furrows, which have not been
produced by weathering.

Some of the finest streaks of colour in the stony layers, alternating
with the obsidian, can be distinctly seen to be due to an incipient
crystallisation of the constituent minerals. The extent to which the
minerals have crystallised can, also, be distinctly seen to be
connected with the greater or less size, and with the number, of the
minute, flattened, crenulated air-cavities or fissures. Numerous facts,
as in the case of geodes, and of cavities in silicified wood, in
primary rocks, and in veins, show that crystallisation is much favoured
by space. Hence, I conclude, that, if in a mass of cooling volcanic
rock, any cause produced in parallel planes a number of minute fissures
or zones of less tension (which from the pent-up vapours would often be
expanded into crenulated air-cavities), the crystallisation of the
constituent parts, and probably the formation of concretions, would be
superinduced or much favoured in such planes; and thus, a laminated
structure of the kind we are here considering would be generated.

That some cause does produce parallel zones of less tension in volcanic
rocks, during their consolidation, we must admit in the case of the
thin alternate layers of obsidian and pumice described by Humboldt, and
of the small, flattened, crenulated air-cells in the laminated rocks of
Ascension; for on no other principle can we conceive why the confined
vapours should through their expansion form air-cells or fibres in
separate, parallel planes, instead of irregularly throughout the mass.
In Mr. Stokes’ collection, I have seen a beautiful example of this
structure, in a specimen of obsidian from Mexico, which is shaded and
zoned, like the finest agate, with numerous, straight, parallel layers,
more or less opaque and white, or almost perfectly glassy; the degree
of opacity and glassiness depending on the number of microscopically
minute, flattened air-cells; in this case, it is scarcely possible to
doubt but that the mass, to which the fragment belonged, must have been
subjected to some, probably prolonged, action, causing the tension
slightly to vary in the successive planes.

Several causes appear capable of producing zones of different tension,
in masses semi-liquified by heat. In a fragment of devitrified glass, I
have observed layers of sphaerulites which appeared, from the manner in
which they were abruptly bent, to have been produced by the simple
contraction of the mass in the vessel, in which it cooled. In certain
dikes on Mount Etna, described by M. Elie de Beaumont (“Mem. pour
servir” etc. tome 4 page 131.), as bordered by alternating bands of
scoriaceous and compact rock, one is led to suppose that the stretching
movement of the surrounding strata, which originally produced the
fissures, continued whilst the injected rock remained fluid. Guided,
however, by Professor Forbes’ (“Edinburgh New Phil. Journal” 1842 page
350.) clear description of the zoned structure of glacier-ice, far the
most probable explanation of the laminated structure of these
feldspathic rocks appears to be, that they have been stretched whilst
slowly flowing onwards in a pasty condition (I presume that this is
nearly the same explanation which Mr. Scrope had in his mind, when he
speaks (“Geolog. Transact.” volume 2 second series page 228) of the
ribboned structure of his trachytic rocks, having arisen, from “a
linear extension of the mass, while in a state of imperfect liquidity,
coupled with a concretionary process.”), in precisely the same manner
as Professor Forbes believes, that the ice of moving glaciers is
stretched and fissured. In both cases, the zones may be compared to
those in the finest agates; in both, they extend in the direction in
which the mass has flowed, and those exposed on the surface are
generally vertical: in the ice, the porous laminae are rendered
distinct by the subsequent congelation of infiltrated water, in the
stony feldspathic lavas, by subsequent crystalline and concretionary
action. The fragment of glassy obsidian in Mr. Stokes’ collection,
which is zoned with minute air-cells must strikingly resemble, judging
from Professor Forbes’ descriptions, a fragment of the zoned ice; and
if the rate of cooling and nature of the mass had been favourable to
its crystallisation or to concretionary action, we should here have had
the finest parallel zones of different composition and texture. In
glaciers, the lines of porous ice and of minute crevices seem to be due
to an incipient stretching, caused by the central parts of the frozen
stream moving faster than the sides and bottom, which are retarded by
friction: hence in glaciers of certain forms and towards the lower end
of most glaciers, the zones become horizontal. May we venture to
suppose that in the feldspathic lavas with horizontal laminae, we see
an analogous case? All geologists, who have examined trachytic regions,
have come to the conclusion, that the lavas of this series have
possessed an exceedingly imperfect fluidity; and as it is evident that
only matter thus characterised would be subject to become fissured and
to be formed into zones of different tensions, in the manner here
supposed, we probably see the reason why augitic lavas, which appear
generally to have possessed a high degree of fluidity, are not, like
the feldspathic lavas, divided into laminae of different composition
and texture. (Basaltic lavas, and many other rocks, are not
unfrequently divided into thick laminae or plates, of the same
composition, which are either straight or curved; these being crossed
by vertical lines of fissure, sometimes become united into columns.
This structure seems related, in its origin, to that by which many
rocks, both igneous and sedimentary, become traversed by parallel
systems of fissures.) Moreover, in the augitic series, there never
appears to be any tendency to concretionary action, which we have seen
plays an important part in the lamination of rocks, of the trachytic
series, or at least in rendering that structure apparent.

Whatever may be thought of the explanation here advanced of the
laminated structure of the rocks of the trachytic series, I venture to
call the attention of geologists to the simple fact, that in a body of
rock at Ascension, undoubtedly of volcanic origin, layers often of
extreme tenuity, quite straight, and parallel to each other, have been
produced;—some composed of distinct crystals of quartz and diopside,
mingled with amorphous augitic specks and granular feldspar,—others
entirely composed of these black augitic specks, with granules of oxide
of iron,—and lastly, others formed of crystalline feldspar, in a more
or less perfect state of purity, together with numerous crystals of
feldspar, placed lengthways. At this island, there is reason to
believe, and in some analogous cases, it is certainly known, that the
laminae have originally been formed with their present high
inclination. Facts of this nature are manifestly of importance, with
relation to the structural origin of that grand series of plutonic
rocks, which like the volcanic have undergone the action of heat, and
which consist of alternate layers of quartz, feldspar, mica and other
minerals.




CHAPTER IV.
ST. HELENA.


Lavas of the feldspathic, basaltic, and submarine series. Section of
Flagstaff Hill and of the Barn. Dikes. Turk’s Cap and Prosperous Bays.
Basaltic ring. Central crateriform ridge, with an internal ledge and a
parapet. Cones of phonolite. Superficial beds of calcareous sandstone.
Extinct land-shells. Beds of detritus. Elevation of the land.
Denudation. Craters of elevation.

The whole island is of volcanic origin; its circumference, according to
Beatson, is about twenty-eight miles. (Governor Beatson “Account of St.
Helena.”) The central and largest part consists of rocks of a
feldspathic nature, generally decomposed to an extraordinary degree;
and when in this state, presenting a singular assemblage of
alternating, red, purple, brown, yellow, and white, soft, argillaceous
beds. From the shortness of our visit, I did not examine these beds
with care; some of them, especially those of the white, yellow, and
brown shades, originally existed as streams of lava, but the greater
number were probably ejected in the form of scoriae and ashes: other
beds of a purple tint, porphyritic with crystal- shaped patches of a
white, soft substance, which are now unctuous, and yield, like wax, a
polished streak to the nail, seem once to have existed as solid
claystone-porphyries: the red argillaceous beds generally have a
brecciated structure, and no doubt have been formed by the
decomposition of scoriae. Several extensive streams, however, belonging
to this series, retain their stony character; these are either of a
blackish-green colour, with minute acicular crystals of feldspar, or of
a very pale tint, and almost composed of minute, often scaly, crystals
of feldspar, abounding with microscopical black specks; they are
generally compact and laminated; others, however, of similar
composition, are cellular and somewhat decomposed. None of these rocks
contain large crystals of feldspar, or have the harsh fracture peculiar
to trachyte. These feldspathic lavas and tuffs are the uppermost or
those last erupted; innumerable dikes, however, and great masses of
molten rock, have subsequently been injected into them. They converge,
as they rise, towards the central curved ridge, of which one point
attains the elevation of 2,700 feet. This ridge is the highest land in
the island; and it once formed the northern rim of a great crater,
whence the lavas of this series flowed: from its ruined condition, from
the southern half having been removed, and from the violent dislocation
which the whole island has undergone, its structure is rendered very
obscure.

BASALTIC SERIES.

The margin of the island is formed by a rude circle of great, black,
stratified, ramparts of basalt, dipping seaward, and worn into cliffs,
which are often nearly perpendicular, and vary in height from a few
hundred feet to two thousand. This circle, or rather horse-shoe shaped
ring, is open to the south, and is breached by several other wide
spaces. Its rim or summit generally projects little above the level of
the adjoining inland country; and the more recent feldspathic lavas,
sloping down from the central heights, generally abut against and
overlap its inner margin; on the north-western side of the island,
however, they appear (judging from a distance) to have flowed over and
concealed portions of it. In some parts, where the basaltic ring has
been breached, and the black ramparts stand detached, the feldspathic
lavas have passed between them, and now overhang the sea-coast in lofty
cliffs. The basaltic rocks are of a black colour and thinly stratified;
they are generally highly vesicular, but occasionally compact; some of
them contain numerous crystals of glassy feldspar and octahedrons of
titaniferous iron; others abound with crystals of augite and grains of
olivine. The vesicles are frequently lined with minute crystals (of
chabasie?) and even become amygdaloidal with them. The streams are
separated from each other by cindery matter, or by a bright red,
friable, saliferous tuff, which is marked by successive lines like
those of aqueous deposition; and sometimes it has an obscure,
concretionary structure. The rocks of this basaltic series occur
nowhere except near the coast. In most volcanic districts the trachytic
lavas are of anterior origin to the basaltic; but here we see, that a
great pile of rock, closely related in composition to the trachytic
family, has been erupted subsequently to the basaltic strata: the
number, however, of dikes, abounding with large crystals of augite,
with which the feldspathic lavas have been injected, shows perhaps some
tendency to a return to the more usual order of superposition.

BASAL SUBMARINE LAVAS.

The lavas of this basal series lie immediately beneath both the
basaltic and feldspathic rocks. According to Mr. Seale, they may be
seen at intervals on the sea-beach round the entire island. (“Geognosy
of the Island of St. Helena.” Mr. Seale has constructed a gigantic
model of St. Helena, well worth visiting, which is now deposited at
Addiscombe College, in Surrey.) In the sections which I examined, their
nature varied much; some of the strata abound with crystals of augite;
others are of a brown colour, either laminated or in a rubbly
condition; and many parts are highly amygdaloidal with calcareous
matter. The successive sheets are either closely united together, or
are separated from each other by beds of scoriaceous rock and of
laminated tuff, frequently containing well-rounded fragments. The
interstices of these beds are filled with gypsum and salt; the gypsum
also sometimes occurring in thin layers. From the large quantity of
these two substances, from the presence of rounded pebbles in the
tuffs, and from the abundant amygdaloids, I cannot doubt that these
basal volcanic strata flowed beneath the sea. This remark ought perhaps
to be extended to a part of the superincumbent basaltic rocks; but on
this point, I was not able to obtain clear evidence. The strata of the
basal series, whenever I examined them, were intersected by an
extraordinary number of dikes.

FLAGSTAFF HILL AND THE BARN.

(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East
(right)) Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high.

The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata; the dikes
are shaded transversely.)

I will now describe some of the more remarkable sections, and will
commence with these two hills, which form the principal external
feature on the north-eastern side of the island. The square, angular
outline, and black colour of the Barn, at once show that it belongs to
the basaltic series; whilst the smooth, conical figure, and the varied
bright tints of Flagstaff Hill, render it equally clear, that it is
composed of the softened, feldspathic rocks. These two lofty hills are
connected (as is shown in Figure 8) by a sharp ridge, which is composed
of the rubbly lavas of the basal series. The strata of this ridge dip
westward, the inclination becoming less and less towards the Flagstaff;
and the upper feldspathic strata of this hill can be seen, though with
some difficulty, to dip conformably to the W.S.W. Close to the Barn,
the strata of the ridge are nearly vertical, but are much obscured by
innumerable dikes; under this hill, they probably change from being
vertical into being inclined into an opposite direction; for the upper
or basaltic strata, which are about eight hundred or one thousand feet
in thickness, are inclined north-eastward, at an angle between thirty
and forty degrees.

This ridge, and likewise the Barn and Flagstaff Hills, are interlaced
by dikes, many of which preserve a remarkable parallelism in a N.N.W.
and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic
with large crystals of augite; others are formed of a fine-grained and
brown- coloured trap. Most of these dikes are coated by a glossy layer,
from one to two-tenths of an inch in thickness, which, unlike true
pitchstone, fuses into a black enamel; this layer is evidently
analogous to the glossy superficial coating of many lava streams. (This
circumstance has been observed (Lyell “Principles of Geology” volume 4
chapter 10 page 9) in the dikes of the Atrio del Cavallo, but
apparently it is not of very common occurrence. Sir G. Mackenzie,
however, states (page 372 “Travels in Iceland”) that all the veins in
Iceland have a “black vitreous coating on their sides.” Captain
Carmichael, speaking of the dikes in Tristan d’Acunha, a volcanic
island in the Southern Atlantic, says (“Linnaean Transactions” volume
12 page 485) that their sides, “where they come in contact with the
rocks, are invariably in a semi-vitrified state.”) The dikes can often
be followed for great lengths both horizontally and vertically, and
they seem to preserve a nearly uniform thickness (“Geognosy of the
Island of St. Helena” plate 5.): Mr. Seale states, that one near the
Barn, in a height of 1,260 feet, decreases in width only four
inches,—from nine feet at the bottom, to eight feet and eight inches at
the top. On the ridge, the dikes appear to have been guided in their
course, to a considerable degree, by the alternating soft and hard
strata: they are often firmly united to the harder strata, and they
preserve their parallelism for such great lengths, that in very many
instances it was impossible to conjecture, which of the beds were
dikes, and which streams of lava. The dikes, though so numerous on this
ridge, are even more numerous in the valleys a little south of it, and
to a degree I never saw equalled anywhere else: in these valleys they
extend in less regular lines, covering the ground with a network, like
a spider’s web, and with some parts of the surface even appearing to
consist wholly of dikes, interlaced by other dikes.

From the complexity produced by the dikes, from the high inclination
and anticlinal dip of the strata of the basal series, which are
overlaid, at the opposite ends of the short ridge, by two great masses
of different ages and of different composition, I am not surprised that
this singular section has been misunderstood. It has even been supposed
to form part of a crater; but so far is this from having been the case,
that the summit of Flagstaff Hill once formed the lower extremity of a
sheet of lava and ashes, which were erupted from the central,
crateriform ridge. Judging from the slope of the contemporaneous
streams in an adjoining and undisturbed part of the island, the strata
of the Flagstaff Hill must have been upturned at least twelve hundred
feet, and probably much more, for the great truncated dikes on its
summit show that it has been largely denuded. The summit of this hill
now nearly equals in height the crateriform ridge; and before having
been denuded, it was probably higher than this ridge, from which it is
separated by a broad and much lower tract of country; we here,
therefore, see that the lower extremities of a set of lava-streams have
been tilted up to as great a height as, or perhaps greater height than,
the crater, down the flanks of which they originally flowed. I believe
that dislocations on so grand a scale are extremely rare in volcanic
districts. (M. Constant Prevost “Mem. de la Soc. Geolog.” tome 2
observes that “les produits volcaniques n’ont que localement et
rarement meme derange le sol, a travers lequel ils se sont fait jour.”)
The formation of such numbers of dikes in this part of the island shows
that the surface must here have been stretched to a quite extraordinary
degree: this stretching, on the ridge between Flagstaff and Barn Hills,
probably took place subsequently (though perhaps immediately so) to the
strata being tilted; for had the strata at that time extended
horizontally, they would in all probability have been fissured and
injected transversely, instead of in the planes of their
stratification. Although the space between the Barn and Flagstaff Hill
presents a distinct anticlinal line extending north and south, and
though most of the dikes range with much regularity in the same line,
nevertheless, at only a mile due south of the ridge the strata lie
undisturbed. Hence the disturbing force seems to have acted under a
point, rather than along a line. The manner in which it has acted, is
probably explained by the structure of Little Stony-top, a mountain
2,000 feet high, situated a few miles southward of the Barn; we there
see, even from a distance, a dark-coloured, sharp, wedge of compact
columnar rock, with the bright-coloured feldspathic strata, sloping
away on each side from its uncovered apex. This wedge, from which it
derives its name of Stony-top, consists of a body of rock, which has
been injected whilst liquified into the overlying strata; and if we may
suppose that a similar body of rock lies injected, beneath the ridge
connecting the Barn and Flagstaff, the structure there exhibited would
be explained.

TURK’S CAP AND PROSPEROUS BAYS.

(FIGURE 9. PROSPEROUS HILL AND THE BARN. (Section S.S.E. (left) to
N.N.W. (right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill
to The Barn.

The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata.)

Prosperous Hill is a great, black, precipitous mountain, situated two
miles and a half south of the Barn, and composed, like it, of basaltic
strata. These rest, in one part, on the brown-coloured, porphyritic
beds of the basal series, and in another part, on a fissured mass of
highly scoriaceous and amygdaloidal rock, which seems to have formed a
small point of eruption beneath the sea, contemporaneously with the
basal series. Prosperous Hill, like the Barn, is traversed by many
dikes, of which the greater number range north and south, and its
strata dip, at an angle of about 20 degrees, rather obliquely from the
island towards the sea. The space between Prosperous Hill and the Barn,
as represented in Figure 9, consists of lofty cliffs, composed of the
lavas of the upper or feldspathic series, which rest, though
unconformably, on the basal submarine strata, as we have seen that they
do at Flagstaff Hill. Differently, however, from in that hill, these
upper strata are nearly horizontal, gently rising towards the interior
of the island; and they are composed of greenish-black, or more
commonly, pale brown, compact lavas, instead of softened and highly
coloured matter. These brown-coloured, compact lavas, consist almost
entirely of small glimmering scales, or of minute acicular crystals, of
feldspar, placed close by the side of each other, and abounding with
minute black specks, apparently of hornblende. The basaltic strata of
Prosperous Hill project only a little above the level of the
gently-sloping, feldspathic streams, which wind round and abut against
their upturned edges. The inclination of the basaltic strata seems to
be too great to have been caused by their having flowed down a slope,
and they must have been tilted into their present position before the
eruption of the feldspathic streams.

BASALTIC RING.

Proceeding round the Island, the lavas of the upper series, southward
of Prosperous Hill, overhang the sea in lofty precipices. Further on,
the headland, called Great Stony-top, is composed, as I believe, of
basalt; as is Long Range Point, on the inland side of which the
coloured beds abut. On the southern side of the island, we see the
basaltic strata of the South Barn, dipping obliquely seaward at a
considerable angle; this headland, also, stands a little above the
level of the more modern, feldspathic lavas. Further on, a large space
of coast, on each side of Sandy Bay, has been much denuded, and there
seems to be left only the basal wreck of the great, central crater. The
basaltic strata reappear, with their seaward dip, at the foot of the
hill, called Man-and-Horse; and thence they are continued along the
whole north-western coast to Sugar-Loaf Hill, situated near to the
Flagstaff; and they everywhere have the same seaward inclination, and
rest, in some parts at least, on the lavas of the basal series. We thus
see that the circumference of the island is formed by a much-broken
ring, or rather, a horse-shoe, of basalt, open to the south, and
interrupted on the eastern side by many wide breaches. The breadth of
this marginal fringe on the north-western side, where alone it is at
all perfect, appears to vary from a mile to a mile and a half. The
basaltic strata, as well as those of the subjacent basal series, dip,
with a moderate inclination, where they have not been subsequently
disturbed, towards the sea. The more broken state of the basaltic ring
round the eastern half, compared with the western half of the island,
is evidently due to the much greater denuding power of the waves on the
eastern or windward side, as is shown by the greater height of the
cliffs on that side, than to leeward. Whether the margin of basalt was
breached, before or after the eruption of the lavas of the upper
series, is doubtful; but as separate portions of the basaltic ring
appear to have been tilted before that event, and from other reasons,
it is more probable, that some at least of the breaches were first
formed. Reconstructing in imagination, as far as is possible, the ring
of basalt, the internal space or hollow, which has since been filled up
with the matter erupted from the great central crater, appears to have
been of an oval figure, eight or nine miles in length by about four
miles in breadth, and with its axis directed in a N.E. and S W. line,
coincident with the present longest axis of the island.

THE CENTRAL CURVED RIDGE.

This ridge consists, as before remarked, of grey feldspathic lavas, and
of red, brecciated, argillaceous tuffs, like the beds of the upper
coloured series. The grey lavas contain numerous, minute, black, easily
fusible specks; and but very few large crystals of feldspar. They are
generally much softened; with the exception of this character, and of
being in many parts highly cellular, they are quite similar to those
great sheets of lava which overhang the coast at Prosperous Bay.
Considerable intervals of time appear to have elapsed, judging from the
marks of denudation, between the formation of the successive beds, of
which this ridge is composed. On the steep northern slope, I observed
in several sections a much worn undulating surface of red tuff, covered
by grey, decomposed, feldspathic lavas, with only a thin earthy layer
interposed between them. In an adjoining part, I noticed a trap-dike,
four feet wide, cut off and covered up by the feldspathic lava, as is
represented in Figure 9. The ridge ends on the eastern side in a hook,
which is not represented clearly enough in any map which I have seen;
towards the western end, it gradually slopes down and divides into
several subordinate ridges. The best defined portion between Diana’s
Peak and Nest Lodge, which supports the highest pinnacles in the island
varying from 2,000 to 2,700 feet, is rather less than three miles long
in a straight line. Throughout this space the ridge has a uniform
appearance and structure; its curvature resembles that of the
coast-line of a great bay, being made up of many smaller curves, all
open to the south. The northern and outer side is supported by narrow
ridges or buttresses, which slope down to the adjoining country. The
inside is much steeper, and is almost precipitous; it is formed of the
basset edges of the strata, which gently decline outwards. Along some
parts of the inner side, a little way beneath the summit, a flat ledge
extends, which imitates in outline the smaller curvatures of the crest.
Ledges of this kind occur not unfrequently within volcanic craters, and
their formation seems to be due to the sinking down of a level sheet of
hardened lava, the edges of which remain (like the ice round a pool,
from which the water has been drained) adhering to the sides. (A most
remarkable instance of this structure is described in Ellis “Polynesian
Researches” second edition where an admirable drawing is given of the
successive ledges or terraces, on the borders of the immense crater at
Hawaii, in the Sandwich Islands.)

(FIGURE 10. DIKE. (Section showing layers 1, 2 and 3 from top to
bottom.)

1. Grey feldspathic lava.

2. A layer, one inch in thickness, of a reddish earthy matter.

3. Brecciated, red, argillaceous tuff.)

In some parts, the ridge is surmounted by a wall or parapet,
perpendicular on both sides. Near Diana’s Peak this wall is extremely
narrow. At the Galapagos Archipelago I observed parapets, having a
quite similar structure and appearance, surmounting several of the
craters; one, which I more particularly examined, was composed of
glossy, red scoriae firmly cemented together; being externally
perpendicular, and extending round nearly the whole circumference of
the crater, it rendered it almost inaccessible. The Peak of Teneriffe
and Cotopaxi, according to Humboldt, are similarly constructed; he
states that “at their summits a circular wall surrounds the crater,
which wall, at a distance, has the appearance of a small cylinder
placed on a truncated cone. (“Personal Narrative” volume 1 page 171.)
On Cotopaxi this peculiar structure is visible to the naked eye at more
than two thousand toises’ distance; and no person has ever reached its
crater. (Humboldt “Picturesque Atlas” folio plate 10.) On the Peak of
Teneriffe, the parapet is so high, that it would be impossible to reach
the caldera, if on the eastern side there did not exist a breach.” The
origin of these circular parapets is probably due to the heat or
vapours from the crater, penetrating and hardening the sides to a
nearly equal depth, and afterwards to the mountain being slowly acted
on by the weather, which would leave the hardened part, projecting in
the form of a cylinder or circular parapet.

From the points of structure in the central ridge, now
enumerated,—namely, from the convergence towards it of the beds of the
upper series,—from the lavas there becoming highly cellular,—from the
flat ledge, extending along its inner and precipitous side, like that
within some still active craters,—from the parapet-like wall on its
summit,—and lastly, from its peculiar curvature, unlike that of any
common line of elevation, I cannot doubt that this curved ridge forms
the last remnant of a great crater. In endeavouring, however, to trace
its former outline, one is soon baffled; its western extremity
gradually slopes down, and, branching into other ridges, extends to the
sea-coast; the eastern end is more curved, but it is only a little
better defined. Some appearances lead me to suppose that the southern
wall of the crater joined the present ridge near Nest Lodge; in this
case the crater must have been nearly three miles long, and about a
mile and a half in breadth. Had the denudation of the ridge and the
decomposition of its constituent rocks proceeded a few steps further,
and had this ridge, like several other parts of the island, been broken
up by great dikes and masses of injected matter, we should in vain have
endeavoured to discover its true nature. Even now we have seen that at
Flagstaff Hill the lower extremity and most distant portion of one
sheet of the erupted matter has been upheaved to as great a height as
the crater down which it flowed, and probably even to a greater height.
It is interesting thus to trace the steps by which the structure of a
volcanic district becomes obscured, and finally obliterated: so near to
this last stage is St. Helena, that I believe no one has hitherto
suspected that the central ridge or axis of the island is the last
wreck of the crater, whence the most modern volcanic streams were
poured forth.

The great hollow space or valley southward of the central curved ridge,
across which the half of the crater must once have extended, is formed
of bare, water-worn hillocks and ridges of red, yellow, and brown
rocks, mingled together in chaos-like confusion, interlaced by dikes,
and without any regular stratification. The chief part consists of red
decomposing scoriae, associated with various kinds of tuff and yellow
argillaceous beds, full of broken crystals, those of augite being
particularly large. Here and there masses of highly cellular and
amygdaloidal lavas protrude. From one of the ridges in the midst of the
valley, a conical precipitous hill, called Lot, boldly stands up, and
forms a most singular and conspicuous object. It is composed of
phonolite, divided in one part into great curved laminae, in another,
into angular concretionary balls, and in a third part into outwardly
radiating columns. At its base the strata of lava, tuff, and scoriae,
dip away on all sides (Abich in his “Views of Vesuvius” plate 6 has
shown the manner in which beds, under nearly similar circumstances, are
tilted up. The upper beds are more turned up than the lower; and he
accounts for this, by showing that the lava insinuates itself
horizontally between the lower beds.); the uncovered portion is 197
feet in height (This height is given by Mr. Seale in his Geognosy of
the island. The height of the summit above the level of the sea is said
to be 1,444 feet.), and its horizontal section gives an oval figure.
The phonolite is of a greenish-grey colour, and is full of minute
acicular crystals of feldspar; in most parts it has a conchoidal
fracture, and is sonorous, yet it is crenulated with minute
air-cavities. In a S.W. direction from Lot, there are some other
remarkable columnar pinnacles, but of a less regular shape, namely,
Lot’s Wife, and the Asses’ Ears, composed of allied kinds of rock. From
their flattened shape, and their relative position to each other, they
are evidently connected on the same line of fissure. It is, moreover,
remarkable that this same N.E. and S.W. line, joining Lot and Lot’s
Wife, if prolonged would intersect Flagstaff Hill, which, as before
stated, is crossed by numerous dikes running in this direction, and
which has a disturbed structure, rendering it probable that a great
body of once fluid rock lies injected beneath it.

In this same great valley there are several other conical masses of
injected rock (one, I observed, was composed of compact greenstone),
some of which are not connected, as far as is apparent, with any line
of dike; whilst others are obviously thus connected. Of these dikes,
three or four great lines stretch across the valley in a N.E. and S.W.
direction, parallel to that one connecting the Asses’ Ears, Lot’s Wife,
and probably Lot. The number of these masses of injected rock is a
remarkable feature in the geology of St. Helena. Besides those just
mentioned, and the hypothetical one beneath Flagstaff Hill, there is
Little Stony-top and others, as I have reason to believe, at the
Man-and-Horse, and at High Hill. Most of these masses, if not all of
them, have been injected subsequently to the last volcanic eruptions
from the central crater. The formation of conical bosses of rock on
lines of fissure, the walls of which are in most cases parallel, may
probably be attributed to inequalities in the tension, causing small
transverse fissures, and at these points of intersection the edges of
the strata would naturally yield, and be easily turned upwards.
Finally, I may remark, that hills of phonolite everywhere are apt to
assume singular and even grotesque shapes, like that of Lot
(D’Aubuisson in his “Traite de Geognosie” tome 2 page 540 particularly
remarks that this is the case.): the peak at Fernando Noronha offers an
instance; at St. Jago, however, the cones of phonolite, though
tapering, have a regular form. Supposing, as seems probable, that all
such hillocks or obelisks have originally been injected, whilst
liquified, into a mould formed by yielding strata, as certainly has
been the case with Lot, how are we to account for the frequent
abruptness and singularity of their outlines, compared with similarly
injected masses of greenstone and basalt? Can it be due to a less
perfect degree of fluidity, which is generally supposed to be
characteristic of the allied trachytic lavas?

SUPERFICIAL DEPOSITS.

Soft calcareous sandstone occurs in extensive, though thin, superficial
beds, both on the northern and southern shores of the island. It
consists of very minute, equal-sized, rounded particles of shells, and
other organic bodies, which partially retain their yellow, brown, and
pink colours, and occasionally, though very rarely, present an obscure
trace of their original external forms. I in vain endeavoured to find a
single unrolled fragment of a shell. The colour of the particles is the
most obvious character by which their origin can be recognised, the
tints being affected (and an odour produced) by a moderate heat, in the
same manner as in fresh shells. The particles are cemented together,
and are mingled with some earthy matter: the purest masses, according
to Beatson, contain 70 per cent of carbonate of lime. The beds, varying
in thickness from two or three feet to fifteen feet, coat the surface
of the ground; they generally lie on that side of the valley which is
protected from the wind, and they occur at the height of several
hundred feet above the level of the sea. Their position is the same
which sand, if now drifted by the trade-wind, would occupy; and no
doubt they thus originated, which explains the equal size and
minuteness of the particles, and likewise the entire absence of whole
shells, or even of moderately-sized fragments. It is remarkable that at
the present day there are no shelly beaches on any part of the coast,
whence calcareous dust could be drifted and winnowed; we must,
therefore, look back to a former period when before the land was worn
into the present great precipices, a shelving coast, like that of
Ascension, was favourable to the accumulation of shelly detritus. Some
of the beds of this limestone are between six hundred and seven hundred
feet above the sea; but part of this height may possibly be due to an
elevation of the land, subsequent to the accumulation of the calcareous
sand.

The percolation of rain-water has consolidated parts of these beds into
a solid rock, and has formed masses of dark brown, stalagmitic
limestone. At the Sugar-Loaf quarry, fragments of rock on the adjoining
slopes have been thickly coated by successive fine layers of calcareous
matter. (In the earthy detritus on several parts of this hill,
irregular masses of very impure, crystallised sulphate of lime occur.
As this substance is now being abundantly deposited by the surf at
Ascension, it is possible that these masses may thus have originated;
but if so, it must have been at a period when the land stood at a much
lower level. This earthy selenite is now found at a height of between
six hundred and seven hundred feet.) It is singular, that many of these
pebbles have their entire surfaces coated, without any point of contact
having been left uncovered; hence, these pebbles must have been lifted
up by the slow deposition between them of the successive films of
carbonate of lime. Masses of white, finely oolitic rock are attached to
the outside of some of these coated pebbles. Von Buch has described a
compact limestone at Lanzarote, which seems perfectly to resemble the
stalagmitic deposition just mentioned: it coats pebbles, and in parts
is finely oolitic: it forms a far-extended layer, from one inch to two
or three feet in thickness, and it occurs at the height of 800 feet
above the sea, but only on that side of the island exposed to the
violent north-western winds. Von Buch remarks, that it is not found in
hollows, but only on the unbroken and inclined surfaces of the
mountain. (“Description des Isles Canaries” page 293.) He believes,
that it has been deposited by the spray which is borne over the whole
island by these violent winds. It appears, however, to me much more
probable that it has been formed, as at St. Helena, by the percolation
of water through finely comminuted shells: for when sand is blown on a
much-exposed coast, it always tends to accumulate on broad, even
surfaces, which offer a uniform resistance to the winds. At the
neighbouring island, moreover, of Feurteventura, there is an earthy
limestone, which, according to Von Buch, is quite similar to specimens
which he has seen from St. Helena, and which he believes to have been
formed by the drifting of shelly detritus. (Idem pages 314 and 374.)

The upper beds of the limestone, at the above-mentioned quarry on the
Sugar-Loaf Hill, are softer, finer-grained and less pure, than the
lower beds. They abound with fragments of land-shells, and with some
perfect ones; they contain, also, the bones of birds, and the large
eggs, apparently of water-fowl. (Colonel Wilkes, in a catalogue
presented with some specimens to the Geological Society, states that as
many as ten eggs were found by one person. Dr. Buckland has remarked
(“Geolog. Trans.” volume 5 page 474) on these eggs.) It is probable
that these upper beds remained long in an unconsolidated form, during
which time, these terrestrial productions were embedded. Mr. G.R.
Sowerby has kindly examined three species of land-shells, which I
procured from this bed, and has described them in detail. One of them
is a Succinea, identical with a species now living abundantly on the
island; the two others, namely, Cochlogena fossilis and Helix
biplicata, are not known in a recent state: the latter species was also
found in another and different locality, associated with a species of
Cochlogena which is undoubtedly extinct.

BEDS OF EXTINCT LAND-SHELLS.

Land-shells, all of which appear to be species now extinct, occur
embedded in earth, in several parts of the island. The greater number
have been found at a considerable height on Flagstaff Hill. On the N.W.
side of this hill, a rain-channel exposes a section of about twenty
feet in thickness, of which the upper part consists of black vegetable
mould, evidently washed down from the heights above, and the lower part
of less black earth, abounding with young and old shells, and with
their fragments: part of this earth is slightly consolidated by
calcareous matter, apparently due to the partial decomposition of some
of the shells. Mr. Seale, an intelligent resident, who first called
attention to these shells, gave me a large collection from another
locality, where the shells appear to have been embedded in very black
earth. Mr. G.R. Sowerby has examined these shells, and has described
them. There are seven species, namely, one Cochlogena, two species of
the genus Cochlicopa, and four of Helix; none of these are known in a
recent state, or have been found in any other country. The smaller
species were picked out of the inside of the large shells of the
Cochlogena aurisvulpina. This last-mentioned species is in many
respects a very singular one; it was classed, even by Lamarck, in a
marine genus, and having thus been mistaken for a sea-shell, and the
smaller accompanying species having been overlooked, the exact
localities where it was found have been measured, and the elevation of
this island thus deduced! It is very remarkable that all the shells of
this species found by me in one spot, form a distinct variety, as
described by Mr. Sowerby, from those procured from another locality by
Mr. Seale. As this Cochlogena is a large and conspicuous shell, I
particularly inquired from several intelligent countrymen whether they
had ever seen it alive; they all assured me that they had not, and they
would not even believe that it was a land animal: Mr. Seale, moreover,
who was a collector of shells all his life at St. Helena, never met
with it alive. Possibly some of the smaller species may turn out to be
yet living kinds; but, on the other hand, the two land- shells which
are now living on the island in great numbers, do not occur embedded,
as far as is yet known, with the extinct species. I have shown in my
“Journal” (“Journal of Researches” page 582.), that the extinction of
these land-shells possibly may not be an ancient event; as a great
change took place in the state of the island about one hundred and
twenty years ago, when the old trees died, and were not replaced by
young ones, these being destroyed by the goats and hogs, which had run
wild in numbers, from the year 1502. Mr. Seale states, that on
Flagstaff Hill, where we have seen that the embedded land-shells are
especially numerous, traces are everywhere discoverable, which plainly
indicate that it was once thickly clothed with trees; at present not
even a bush grows there. The thick bed of black vegetable mould which
covers the shell-bed, on the flanks of this hill, was probably washed
down from the upper part, as soon as the trees perished, and the
shelter afforded by them was lost.

ELEVATION OF THE LAND.

Seeing that the lavas of the basal series, which are of submarine
origin, are raised above the level of the sea, and at some places to
the height of many hundred feet, I looked out for superficial signs of
the elevation of the land. The bottoms of some of the gorges, which
descend to the coast, are filled up to the depth of about a hundred
feet, by rudely divided layers of sand, muddy clay, and fragmentary
masses; in these beds, Mr. Seale has found the bones of the tropic-bird
and of the albatross; the former now rarely, and the latter never
visiting the island. From the difference between these layers, and the
sloping piles of detritus which rest on them, I suspect that they were
deposited, when the gorges stood beneath the sea. Mr. Seale, moreover,
has shown that some of the fissure- like gorges become, with a concave
outline, gradually rather wider at the bottom than at the top; and this
peculiar structure was probably caused by the wearing action of the
sea, when it entered the lower part of these gorges. (A fissure-like
gorge, near Stony-top, is said by Mr. Seale to be 840 feet deep, and
only 115 feet in width.) At greater heights, the evidence of the rise
of the land is even less clear: nevertheless, in a bay-like depression
on the table-land behind Prosperous Bay, at the height of about a
thousand feet, there are flat-topped masses of rock, which it is
scarcely conceivable, could have been insulated from the surrounding
and similar strata, by any other agency than the denuding action of a
sea- beach. Much denudation, indeed, has been effected at great
elevations, which it would not be easy to explain by any other means:
thus, the flat summit of the Barn, which is 2,000 feet high, presents,
according to Mr. Seale, a perfect network of truncated dikes; on hills
like the Flagstaff, formed of soft rock, we might suppose that the
dikes had been worn down and cut off by meteoric agency, but we can
hardly suppose this possible with the hard, basaltic strata of the
Barn.

COAST DENUDATION.

The enormous cliffs, in many parts between one and two thousand feet in
height, with which this prison-like island is surrounded, with the
exception of only a few places, where narrow valleys descend to the
coast, is the most striking feature in its scenery. We have seen that
portions of the basaltic ring, two or three miles in length by one or
two miles in breadth, and from one to two thousand feet in height, have
been wholly removed. There are, also, ledges and banks of rock, rising
out of profoundly deep water, and distant from the present coast
between three and four miles, which, according to Mr. Seale, can be
traced to the shore, and are found to be the continuations of certain
well-known great dikes. The swell of the Atlantic Ocean has obviously
been the active power in forming these cliffs; and it is interesting to
observe that the lesser, though still great, height of the cliffs on
the leeward and partially protected side of the island (extending from
the Sugar-Loaf Hill to South West Point), corresponds with the lesser
degree of exposure. When reflecting on the comparatively low coasts of
many volcanic islands, which also stand exposed in the open ocean, and
are apparently of considerable antiquity, the mind recoils from an
attempt to grasp the number of centuries of exposure, necessary to have
ground into mud and to have dispersed the enormous cubic mass of hard
rock which has been pared off the circumference of this island. The
contrast in the superficial state of St. Helena, compared with the
nearest island, namely, Ascension, is very striking. At Ascension, the
surfaces of the lava-streams are glossy, as if just poured forth, their
boundaries are well defined, and they can often be traced to perfect
craters, whence they were erupted; in the course of many long walks, I
did not observe a single dike; and the coast round nearly the entire
circumference is low, and has been eaten back (though too much stress
must not be placed on this fact, as the island may have been subsiding)
into a little wall only from ten to thirty feet high. Yet during the
340 years, since Ascension has been known, not even the feeblest signs
of volcanic action have been recorded. (In the “Nautical Magazine” for
1835 page 642, and for 1838 page 361, and in the “Comptes Rendus” April
1838, accounts are given of a series of volcanic
phenomena—earthquakes—troubled water—floating scoriae and columns of
smoke—which have been observed at intervals since the middle of the
last century, in a space of open sea between longitudes 20 degrees and
22 degrees west, about half a degree south of the equator. These facts
seem to show, that an island or an archipelago is in process of
formation in the middle of the Atlantic: a line joining St. Helena and
Ascension, prolonged, intersects this slowly nascent focus of volcanic
action.) On the other hand, at St. Helena, the course of no one stream
of lava can be traced, either by the state of its boundaries or of its
superficies; the mere wreck of one great crater is left; not the
valleys only, but the surfaces of some of the highest hills, are
interlaced by worn-down dikes, and, in many places, the denuded summits
of great cones of injected rock stand exposed and naked; lastly, as we
have seen, the entire circuit of the island has been deeply worn back
into the grandest precipices.

CRATERS OF ELEVATION.

There is much resemblance in structure and in geological history
between St. Helena, St. Jago, and Mauritius. All three islands are
bounded (at least in the parts which I was able to examine) by a ring
of basaltic mountains, now much broken, but evidently once continuous.
These mountains have, or apparently once had, their escarpments steep
towards the interior of the island, and their strata dip outwards. I
was able to ascertain, only in a few cases, the inclination of the
beds; nor was this easy, for the stratification was generally obscure,
except when viewed from a distance. I feel, however, little doubt that,
according to the researches of M. Elie de Beaumont, their average
inclination is greater than that which they could have acquired,
considering their thickness and compactness, by flowing down a sloping
surface. At St. Helena, and at St. Jago, the basaltic strata rest on
older and probably submarine beds of different composition. At all
three islands, deluges of more recent lavas have flowed from the centre
of the island, towards and between the basaltic mountains; and at St.
Helena the central platform has been filled up by them. All three
islands have been raised in mass. At Mauritius the sea, within a late
geological period, must have reached to the foot of the basaltic
mountains, as it now does at St. Helena; and at St. Jago it is cutting
back the intermediate plain towards them. In these three islands, but
especially at St. Jago and at Mauritius, when, standing on the summit
of one of the old basaltic mountains, one looks in vain towards the
centre of the island,—the point towards which the strata beneath one’s
feet, and of the mountains on each side, rudely converge,—for a source
whence these strata could have been erupted; but one sees only a vast
hollow platform stretched beneath, or piles of matter of more recent
origin.

These basaltic mountains come, I presume, into the class of Craters of
elevation: it is immaterial whether the rings were ever completely
formed, for the portions which now exist have so uniform a structure,
that, if they do not form fragments of true craters, they cannot be
classed with ordinary lines of elevation. With respect to their origin,
after having read the works of Mr. Lyell (“Principles of Geology” fifth
edition volume 2 page 171.), and of MM. C. Prevost and Virlet, I cannot
believe that the great central hollows have been formed by a simple
dome-shaped elevation, and the consequent arching of the strata. On the
other hand, I have very great difficulty in admitting that these
basaltic mountains are merely the basal fragments of great volcanoes,
of which the summits have either been blown off, or more probably
swallowed up by subsidence. These rings are, in some instances, so
immense, as at St. Jago and at Mauritius, and their occurrence is so
frequent, that I can hardly persuade myself to adopt this explanation.
Moreover, I suspect that the following circumstances, from their
frequent concurrence, are someway connected together,—a connection not
implied in either of the above views: namely, first, the broken state
of the ring; showing that the now detached portions have been exposed
to great denudation, and in some cases, perhaps, rendering it probable
that the ring never was entire; secondly, the great amount of matter
erupted from the central area after or during the formation of the
ring; and thirdly, the elevation of the district in mass. As far as
relates to the inclination of the strata being greater than that which
the basal fragments of ordinary volcanoes would naturally possess, I
can readily believe that this inclination might have been slowly
acquired by that amount of elevation, of which, according to M. Elie de
Beaumont, the numerous upfilled fissures or dikes are the evidence and
the measure,—a view equally novel and important, which we owe to the
researches of that geologist on Mount Etna.

A conjecture, including the above circumstances, occurred to me, when,—
with my mind fully convinced, from the phenomena of 1835 in South
America, that the forces which eject matter from volcanic orifices and
raise continents in mass are identical,—I viewed that part of the coast
of St. Jago, where the horizontally upraised, calcareous stratum dips
into the sea, directly beneath a cone of subsequently erupted lava. (I
have given a detailed account of these phenomena, in a paper read
before the Geological Society in March 1838. At the instant of time,
when an immense area was convulsed and a large tract elevated, the
districts immediately surrounding several of the great vents in the
Cordillera remained quiescent; the subterranean forces being apparently
relieved by the eruptions, which then recommenced with great violence.
An event of somewhat the same kind, but on an infinitely smaller scale,
appears to have taken place, according to Abich (“Views of Vesuvius”
plates 1 and 9), within the great crater of Vesuvius, where a platform
on one side of a fissure was raised in mass twenty feet, whilst on the
other side, a train of small volcanoes burst forth in eruption.) The
conjecture is that, during the slow elevation of a volcanic district or
island, in the centre of which one or more orifices continue open, and
thus relieve the subterranean forces, the borders are elevated more
than the central area; and that the portions thus upraised do not slope
gently into the central, less elevated area, as does the calcareous
stratum under the cone at St. Jago, and as does a large part of the
circumference of Iceland, but that they are separated from it by curved
faults. (It appears, from information communicated to me in the most
obliging manner by M. E. Robert, that the circumferential parts of
Iceland, which are composed of ancient basaltic strata alternating with
tuff, dip inland, thus forming a gigantic saucer. M. Robert found that
this was the case, with a few and quite local exceptions, for a space
of coast several hundred miles in length. I find this statement
corroborated, as far as regards one place, by Mackenzie in his
“Travels” page 377, and in another place by some MS. notes kindly lent
me by Dr. Holland. The coast is deeply indented by creeks, at the head
of which the land is generally low. M. Robert informs me, that the
inwardly dipping strata appear to extend as far as this line, and that
their inclination usually corresponds with the slope of the surface,
from the high coast-mountains to the low land at the head of these
creeks. In the section described by Sir G. Mackenzie, the dip is 120.
The interior parts of the island chiefly consist, as far as is known,
of recently erupted matter. The great size, however, of Iceland,
equalling the bulkiest part of England, ought perhaps to exclude it
from the class of islands we have been considering; but I cannot avoid
suspecting that if the coast-mountains, instead of gently sloping into
the less elevated central area, had been separated from it by
irregularly curved faults, the strata would have been tilted seaward,
and a “Crater of elevation,” like that of St. Jago or that of
Mauritius, but of much vaster dimensions, would have been formed. I
will only further remark, that the frequent occurrence of extensive
lakes at the foot of large volcanoes, and the frequent association of
volcanic and fresh-water strata, seem to indicate that the areas around
volcanoes are apt to be depressed beneath the level of the adjoining
country, either from having been less elevated, or from the effects of
subsidence.) We might expect, from what we see along ordinary faults,
that the strata on the upraised side, already dipping outwards from
their original formation as lava-streams, would be tilted from the line
of fault, and thus have their inclination increased. According to this
hypothesis, which I am tempted to extend only to some few cases, it is
not probable that the ring would ever be formed quite perfect; and from
the elevation being slow, the upraised portions would generally be
exposed to much denudation, and hence the ring become broken; we might
also expect to find occasional inequalities in the dip of the upraised
masses, as is the case at St. Jago. By this hypothesis the elevation of
the districts in mass, and the flowing of deluges of lava from the
central platforms, are likewise connected together. On this view the
marginal basaltic mountains of the three foregoing islands might still
be considered as forming “Craters of elevation;” the kind of elevation
implied having been slow, and the central hollow or platform having
been formed, not by the arching of the surface, but simply by that part
having been upraised to a less height.




CHAPTER V.
GALAPAGOS ARCHIPELAGO.


Chatham Island. Craters composed of a peculiar kind of tuff. Small
basaltic craters, with hollows at their bases. Albemarle Island; fluid
lavas, their composition. Craters of tuff; inclination of their
exterior diverging strata, and structure of their interior converging
strata. James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected fragments.
Concluding remarks on the craters of tuff, and on the breached
condition of their southern sides. Mineralogical composition of the
rocks of the archipelago. Elevation of the land. Direction of the
fissures of eruption.

(FIGURE 11. MAP 3. GALAPAGOS ARCHIPELAGO.

Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle,
James, Indefatigable, Barrington, Chatham, Charles and Hood’s Islands.)

This archipelago is situated under the equator, at a distance of
between five and six hundred miles from the west coast of South
America. It consists of five principal islands, and of several small
ones, which together are equal in area, but not in extent of land, to
Sicily, conjointly with the Ionian Islands. (I exclude from this
measurement, the small volcanic islands of Culpepper and Wenman, lying
seventy miles northward of the group. Craters were visible on all the
islands of the group, except on Towers Island, which is one of the
lowest; this island is, however, formed of volcanic rocks.) They are
all volcanic: on two, craters have been seen in eruption, and on
several of the other islands, streams of lava have a recent appearance.
The larger islands are chiefly composed of solid rock, and they rise
with a tame outline to a height of between one and four thousand feet.
They are sometimes, but not generally, surmounted by one principal
orifice. The craters vary in size from mere spiracles to huge caldrons
several miles in circumference; they are extraordinarily numerous, so
that I should think, if enumerated, they would be found to exceed two
thousand; they are formed either of scoriae and lava, or of a
brown-coloured tuff; and these latter craters are in several respects
remarkable. The whole group was surveyed by the officers of the
“Beagle.” I visited myself four of the principal islands, and received
specimens from all the others. Under the head of the different islands
I will describe only that which appears to me deserving of attention.

CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF.

Towards the eastern end of this island there occur two craters composed
of two kinds of tuff; one kind being friable, like slightly
consolidated ashes; and the other compact, and of a different nature
from anything which I have met with described. This latter substance,
where it is best characterised, is of a yellowish-brown colour,
translucent, and with a lustre somewhat resembling resin; it is
brittle, with an angular, rough, and very irregular fracture,
sometimes, however, being slightly granular, and even obscurely
crystalline: it can readily be scratched with a knife, yet some points
are hard enough just to mark common glass; it fuses with ease into a
blackish-green glass. The mass contains numerous broken crystals of
olivine and augite, and small particles of black and brown scoriae; it
is often traversed by thin seams of calcareous matter. It generally
affects a nodular or concretionary structure. In a hand specimen, this
substance would certainly be mistaken for a pale and peculiar variety
of pitchstone; but when seen in mass its stratification, and the
numerous layers of fragments of basalt, both angular and rounded, at
once render its subaqueous origin evident. An examination of a series
of specimens shows that this resin-like substance results from a
chemical change on small particles of pale and dark-coloured
scoriaceous rocks; and this change could be distinctly traced in
different stages round the edges of even the same particle. The
position near the coast of all the craters composed of this kind of
tuff or peperino, and their breached condition, renders it probable
that they were all formed when standing immersed in the sea;
considering this circumstance, together with the remarkable absence of
large beds of ashes in the whole archipelago, I think it highly
probable that much the greater part of the tuff has originated from the
trituration of fragments of the grey, basaltic lavas in the mouths of
craters standing in the sea. It may be asked whether the heated water
within these craters has produced this singular change in the small
scoriaceous particles and given to them their translucent, resin-like
fracture. Or has the associated lime played any part in this change? I
ask these questions from having found at St. Jago, in the Cape de Verde
Islands, that where a great stream of molten lava has flowed over a
calcareous bottom into the sea, the outermost film, which in other
parts resembles pitchstone, is changed, apparently by its contact with
the carbonate of lime, into a resin-like substance, precisely like the
best characterised specimens of the tuff from this archipelago. (The
concretions containing lime, which I have described at Ascension, as
formed in a bed of ashes, present some degree of resemblance to this
substance, but they have not a resinous fracture. At St. Helena, also,
I found veins of a somewhat similar, compact, but non- resinous
substance, occurring in a bed of pumiceous ashes, apparently free from
calcareous matter: in neither of these cases could heat have acted.)

To return to the two craters: one of them stands at the distance of a
league from the coast, the intervening tract consisting of a calcareous
tuff, apparently of submarine origin. This crater consists of a circle
of hills some of which stand quite detached, but all have a very
regular, qua- qua versal dip, at an inclination of between thirty and
forty degrees. The lower beds, to the thickness of several hundred
feet, consist of the resin- like stone, with embedded fragments of
lava. The upper beds, which are between thirty and forty feet in
thickness, are composed of a thinly stratified, fine-grained, harsh,
friable, brown-coloured tuff, or peperino. (Those geologists who
restrict the term of “tuff” to ashes of a white colour, resulting from
the attrition of feldspathic lavas, would call these brown-coloured
strata “peperino.”) A central mass without any stratification, which
must formerly have occupied the hollow of the crater, but is now
attached only to a few of the circumferential hills, consists of a
tuff, intermediate in character between that with a resin-like, and
that with an earthy fracture. This mass contains white calcareous
matter in small patches. The second crater (520 feet in height) must
have existed until the eruption of a recent, great stream of lava, as a
separate islet; a fine section, worn by the sea, shows a grand
funnel-shaped mass of basalt, surrounded by steep, sloping flanks of
tuff, having in parts an earthy, and in others a semi-resinous
fracture. The tuff is traversed by several broad, vertical dikes, with
smooth and parallel sides, which I did not doubt were formed of basalt,
until I actually broke off fragments. These dikes, however, consist of
tuff like that of the surrounding strata, but more compact, and with a
smoother fracture; hence we must conclude, that fissures were formed
and filled up with the finer mud or tuff from the crater, before its
interior was occupied, as it now is, by a solidified pool of basalt.
Other fissures have been subsequently formed, parallel to these
singular dikes, and are merely filled with loose rubbish. The change
from ordinary scoriaceous particles to the substance with a
semi-resinous fracture, could be clearly followed in portions of the
compact tuff of these dikes.

(FIGURE 12. THE KICKER ROCK, 400 FEET HIGH.)

At the distance of a few miles from these two craters, stands the
Kicker Rock, or islet, remarkable from its singular form. It is
unstratified, and is composed of compact tuff, in parts having the
resin-like fracture. It is probable that this amorphous mass, like that
similar mass in the case first described, once filled up the central
hollow of a crater, and that its flanks, or sloping walls, have since
been worn quite away by the sea, in which it stands exposed.

SMALL BASALTIC CRATERS.

A bare, undulating tract, at the eastern end of Chatham Island, is
remarkable from the number, proximity, and form of the small basaltic
craters with which it is studded. They consist, either of a mere
conical pile, or, but less commonly, of a circle, of black and red,
glossy scoriae, partially cemented together. They vary in diameter from
thirty to one hundred and fifty yards, and rise from about fifty to one
hundred feet above the level of the surrounding plain. From one small
eminence, I counted sixty of these craters, all of which were within a
third of a mile from each other, and many were much closer. I measured
the distance between two very small craters, and found that it was only
thirty yards from the summit-rim of one to the rim of the other. Small
streams of black, basaltic lava, containing olivine and much glassy
feldspar, have flowed from many, but not from all of these craters. The
surfaces of the more recent streams were exceedingly rugged, and were
crossed by great fissures; the older streams were only a little less
rugged; and they were all blended and mingled together in complete
confusion. The different growth, however, of the trees on the streams,
often plainly marked their different ages. Had it not been for this
latter character, the streams could in few cases have been
distinguished; and, consequently, this wide undulatory tract might have
(as probably many tracts have) been erroneously considered as formed by
one great deluge of lava, instead of by a multitude of small streams,
erupted from many small orifices.

In several parts of this tract, and especially at the base of the small
craters, there are circular pits, with perpendicular sides, from twenty
to forty feet deep. At the foot of one small crater, there were three
of these pits. They have probably been formed, by the falling in of the
roofs of small caverns. (M. Elie de Beaumont has described (“Mem. pour
servir” etc. tome 4 page 113) many “petits cirques d’eboulement” on
Etna, of some of which the origin is historically known.) In other
parts, there are mammiform hillocks, which resemble great bubbles of
lava, with their summits fissured by irregular cracks, which appeared,
upon entering them, to be very deep; lava has not flowed from these
hillocks. There are, also, other very regular, mammiform hillocks,
composed of stratified lava, and surmounted by circular, steep-sided
hollows, which, I suppose have been formed by a body of gas, first,
arching the strata into one of the bubble- like hillocks, and then,
blowing off its summit. These several kinds of hillocks and pits, as
well as the numerous, small, scoriaceous craters, all show that this
tract has been penetrated, almost like a sieve, by the passage of
heated vapours. The more regular hillocks could only have been heaved
up, whilst the lava was in a softened state. (Sir G. Mackenzie “Travels
in Iceland” pages 389 to 392, has described a plain of lava at the foot
of Hecla, everywhere heaved up into great bubbles or blisters. Sir
George states that this cavernous lava composes the uppermost stratum;
and the same fact is affirmed by Von Buch “Descript. des Isles
Canaries” page 159, with respect to the basaltic stream near Rialejo,
in Teneriffe. It appears singular that it should be the upper streams
that are chiefly cavernous, for one sees no reason why the upper and
lower should not have been equally affected at different times;—have
the inferior streams flowed beneath the pressure of the sea, and thus
been flattened, after the passage through them, of bodies of gas?)

ALBEMARLE ISLAND.

This island consists of five, great, flat-topped craters, which,
together with the one on the adjoining island of Narborough, singularly
resemble each other, in form and height. The southern one is 4,700 feet
high, two others are 3,720 feet, a third only 50 feet higher, and the
remaining ones apparently of nearly the same height. Three of these are
situated on one line, and their craters appear elongated in nearly the
same direction. The northern crater, which is not the largest, was
found by the triangulation to measure, externally, no less than three
miles and one-eighth of a mile in diameter. Over the lips of these
great, broad caldrons, and from little orifices near their summits,
deluges of black lava have flowed down their naked sides.

FLUIDITY OF DIFFERENT LAVAS.

Near Tagus or Banks’ Cove, I examined one of these great streams of
lava, which is remarkable from the evidence of its former high degree
of fluidity, especially when its composition is considered. Near the
sea-coast this stream is several miles in width. It consists of a
black, compact base, easily fusible into a black bead, with angular and
not very numerous air-cells, and thickly studded with large, fractured
crystals of glassy albite, varying from the tenth of an inch to half an
inch in diameter. (In the Cordillera of Chile, I have seen lava very
closely resembling this variety at the Galapagos Archipelago. It
contained, however, besides the albite, well-formed crystals of augite,
and the base (perhaps in consequence of the aggregation of the augitic
particles) was a shade lighter in colour. I may here remark, that in
all these cases, I call the feldspathic crystals, “albite,” from their
cleavage-planes (as measured by the reflecting goniometer)
corresponding with those of that mineral. As, however, other species of
this genus have lately been discovered to cleave in nearly the same
planes with albite, this determination must be considered as only
provisional. I examined the crystals in the lavas of many different
parts of the Galapagos group, and I found that none of them, with the
exception of some crystals from one part of James Island, cleaved in
the direction of orthite or potash-feldspar.) This lava, although at
first sight appearing eminently porphyritic, cannot properly be
considered so, for the crystals have evidently been enveloped, rounded,
and penetrated by the lava, like fragments of foreign rock in a
trap-dike. This was very clear in some specimens of a similar lava,
from Abingdon Island, in which the only difference was, that the
vesicles were spherical and more numerous. The albite in these lavas is
in a similar condition with the leucite of Vesuvius, and with the
olivine, described by Von Buch, as projecting in great balls from the
basalt of Lanzarote. (“Description des Isles Canaries” page 295.)
Besides the albite, this lava contains scattered grains of a green
mineral, with no distinct cleavage, and closely resembling olivine
(Humboldt mentions that he mistook a green augitic mineral, occurring
in the volcanic rocks of the Cordillera of Quito, for olivine.); but as
it fuses easily into a green glass, it belongs probably to the augitic
family: at James Island, however, a similar lava contained true
olivine. I obtained specimens from the actual surface, and from a depth
of four feet, but they differed in no respect. The high degree of
fluidity of this lava-stream was at once evident, from its smooth and
gently sloping surface, from the manner in which the main stream was
divided by small inequalities into little rills, and especially from
the manner in which its edges, far below its source, and where it must
have been in some degree cooled, thinned out to almost nothing; the
actual margin consisting of loose fragments, few of which were larger
than a man’s head. The contrast between this margin, and the steep
walls, above twenty feet high, bounding many of the basaltic streams at
Ascension, is very remarkable. It has generally been supposed that
lavas abounding with large crystals, and including angular vesicles,
have possessed little fluidity; but we see that the case has been very
different at Albemarle Island. (The irregular and angular form of the
vesicles is probably caused by the unequal yielding of a mass composed,
in almost equal proportion, of solid crystals and of a viscid base. It
certainly seems a general circumstance, as might have been expected,
that in lava, which has possessed a high degree of fluidity, AS WELL AS
AN EVEN-SIZED GRAIN, the vesicles are internally smooth and spherical.)
The degree of fluidity in different lavas, does not seem to correspond
with any APPARENT corresponding amount of difference in their
composition: at Chatham Island, some streams, containing much glassy
albite and some olivine, are so rugged, that they may be compared to a
sea frozen during a storm; whilst the great stream at Albemarle Island
is almost as smooth as a lake when ruffled by a breeze. At James
Island, black basaltic lava, abounding with small grains of olivine,
presents an intermediate degree of roughness; its surface being glossy,
and the detached fragments resembling, in a very singular manner, folds
of drapery, cables, and pieces of the bark of trees. (A specimen of
basaltic lava, with a few small broken crystals of albite, given me by
one of the officers, is perhaps worthy of description. It consists of
cylindrical ramifications, some of which are only the twentieth of an
inch in diameter, and are drawn out into the sharpest points. The mass
has not been formed like a stalactite, for the points terminate both
upwards and downwards. Globules, only the fortieth of an inch in
diameter, have dropped from some of the points, and adhere to the
adjoining branches. The lava is vesicular, but the vesicles never reach
the surface of the branches, which are smooth and glossy. As it is
generally supposed that vesicles are always elongated in the direction
of the movement of the fluid mass, I may observe, that in these
cylindrical branches, which vary from a quarter to only the twentieth
of an inch in diameter, every air-cell is spherical.)

CRATERS OF TUFF.

About a mile southward of Banks’ Cove, there is a fine elliptic crater,
about five hundred feet in depth, and three-quarters of a mile in
diameter. Its bottom is occupied by a lake of brine, out of which some
little crateriform hills of tuff rise. The lower beds are formed of
compact tuff, appearing like a subaqueous deposit; whilst the upper
beds, round the entire circumference, consist of a harsh, friable tuff,
of little specific gravity, but often containing fragments of rock in
layers. This upper tuff contains numerous pisolitic balls, about the
size of small bullets, which differ from the surrounding matter, only
in being slightly harder and finer grained. The beds dip away very
regularly on all sides, at angles varying, as I found by measurement,
from twenty-five to thirty degrees. The external surface of the crater
slopes at a nearly similar inclination, and is formed by slightly
convex ribs, like those on the shell of a pecten or scallop, which
become broader as they extend from the mouth of the crater to its base.
These ribs are generally from eight to twenty feet in breadth, but
sometimes they are as much as forty feet broad; and they resemble old,
plastered, much flattened vaults, with the plaster scaling off in
plates: they are separated from each other by gullies, deepened by
alluvial action. At their upper and narrow ends, near the mouth of the
crater, these ribs often consist of real hollow passages, like, but
rather smaller than, those often formed by the cooling of the crust of
a lava-stream, whilst the inner parts have flowed onward;—of which
structure I saw many examples at Chatham Island. There can be no doubt
but that these hollow ribs or vaults have been formed in a similar
manner, namely, by the setting or hardening of a superficial crust on
streams of mud, which have flowed down from the upper part of the
crater. In another part of this same crater, I saw open concave gutters
between one and two feet wide, which appear to have been formed by the
hardening of the lower surface of a mud stream, instead of, as in the
former case, of the upper surface. From these facts I think it is
certain that the tuff must have flowed as mud. (This conclusion is of
some interest, because M. Dufrenoy “Mem. pour servir” tome 4 page 274,
has argued from strata of tuff, apparently of similar composition with
that here described, being inclined at angles between 18 degrees and 20
degrees, that Monte Nuevo and some other craters of Southern Italy have
been formed by upheaval. From the facts given above, of the vaulted
character of the separate rills, and from the tuff not extending in
horizontal sheets round these crateriform hills, no one will suppose
that the strata have here been produced by elevation; and yet we see
that their inclination is above 20 degrees, and often as much as 30
degrees. The consolidated strata also, of the internal talus, as will
be immediately seen, dips at an angle of above 30 degrees.) This mud
may have been formed either within the crater, or from ashes deposited
on its upper parts, and afterwards washed down by torrents of rain. The
former method, in most of the cases, appears the more probable one; at
James Island, however, some beds of the friable kind of tuff extend so
continuously over an uneven surface, that probably they were formed by
the falling of showers of ashes.

Within this same crater, strata of coarse tuff, chiefly composed of
fragments of lava, abut, like a consolidated talus, against the inside
walls. They rise to a height of between one hundred and one hundred and
fifty feet above the surface of the internal brine-lake; they dip
inwards, and are inclined at an angle varying from thirty to thirty-six
degrees. They appear to have been formed beneath water, probably at a
period when the sea occupied the hollow of the crater. I was surprised
to observe that beds having this great inclination did not, as far as
they could be followed, thicken towards their lower extremities.

BANKS’ COVE.

(FIGURE 13. A SECTIONAL SKETCH OF THE HEADLANDS FORMING BANKS’ COVE,
showing the diverging crateriform strata, and the converging stratified
talus. The highest point of these hills is 817 feet above the sea.)

This harbour occupies part of the interior of a shattered crater of
tuff larger than that last described. All the tuff is compact, and
includes numerous fragments of lava; it appears like a subaqueous
deposit. The most remarkable feature in this crater is the great
development of strata converging inwards, as in the last case, at a
considerable inclination, and often deposited in irregular curved
layers. These interior converging beds, as well as the proper,
diverging crateriform strata, are represented in Figure 13, a rude,
sectional sketch of the headlands, forming this Cove. The internal and
external strata differ little in composition, and the former have
evidently resulted from the wear and tear, and redeposition of the
matter forming the external crateriform strata. From the great
development of these inner beds, a person walking round the rim of this
crater might fancy himself on a circular anticlinal ridge of stratified
sandstone and conglomerate. The sea is wearing away the inner and outer
strata, and especially the latter; so that the inwardly converging
strata will, perhaps, in some future age, be left standing alone—a case
which might at first perplex a geologist. (I believe that this case
actually occurs in the Azores, where Dr. Webster “Description” page
185, has described a basin-formed, little island, composed of STRATA OF
TUFF, dipping inwards and bounded externally by steep sea-worn cliffs.
Dr. Daubeny supposes “Volcanoes” page 266, that this cavity must have
been formed by a circular subsidence. It appears to me far more
probable, that we here have strata which were originally deposited
within the hollow of a crater, of which the exterior walls have since
been removed by the sea.)

JAMES ISLAND.

Two craters of tuff on this island are the only remaining ones which
require any notice. One of them lies a mile and a half inland from
Puerto Grande: it is circular, about the third of a mile in diameter,
and 400 feet in depth. It differs from all the other tuff-craters which
I examined, in having the lower part of its cavity, to the height of
between one hundred and one hundred and fifty feet, formed by a
precipitous wall of basalt, giving to the crater the appearance of
having burst through a solid sheet of rock. The upper part of this
crater consists of strata of the altered tuff, with a semi-resinous
fracture. Its bottom is occupied by a shallow lake of brine, covering
layers of salt, which rest on deep black mud. The other crater lies at
the distance of a few miles, and is only remarkable from its size and
perfect condition. Its summit is 1,200 feet above the level of the sea,
and the interior hollow is 600 feet deep. Its external sloping surface
presented a curious appearance from the smoothness of the wide layers
of tuff, which resembled a vast plastered floor. Brattle Island is, I
believe, the largest crater in the Archipelago composed of tuff; its
interior diameter is nearly a nautical mile. At present it is in a
ruined condition, consisting of little more than half a circle open to
the south; its great size is probably due, in part, to internal
degradation, from the action of the sea.

SEGMENT OF A BASALTIC CRATER.

(FIGURE 14. SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach
of Fresh-water Bay.)

One side of Fresh-water Bay, in James Island, is bounded by a
promontory, which forms the last wreck of a great crater. On the beach
of this promontory, a quadrant-shaped segment of a small subordinate
point of eruption stands exposed. It consists of nine separate little
streams of lava piled upon each other; and of an irregular pinnacle,
about fifteen feet high, of reddish-brown, vesicular basalt, abounding
with large crystals of glassy albite, and with fused augite. This
pinnacle, and some adjoining paps of rock on the beach, represent the
axis of the crater. The streams of lava can be followed up a little
ravine, at right angles to the coast, for between ten and fifteen
yards, where they are hidden by detritus: along the beach they are
visible for nearly eighty yards, and I do not believe that they extend
much further. The three lower streams are united to the pinnacle; and
at the point of junction (as shown in Figure 14, a rude sketch made on
the spot), they are slightly arched, as if in the act of flowing over
the lip of the crater. The six upper streams no doubt were originally
united to this same column before it was worn down by the sea. The lava
of these streams is of similar composition with that of the pinnacle,
excepting that the crystals of albite appear to be more comminuted, and
the grains of fused augite are absent. Each stream is separated from
the one above it by a few inches, or at most by one or two feet in
thickness, of loose fragmentary scoriae, apparently derived from the
abrasion of the streams in passing over each other. All these streams
are very remarkable from their thinness. I carefully measured several
of them; one was eight inches thick, but was firmly coated with three
inches above, and three inches below, of red scoriaceous rock (which is
the case with all the streams), making altogether a thickness of
fourteen inches: this thickness was preserved quite uniformly along the
entire length of the section. A second stream was only eight inches
thick, including both the upper and lower scoriaceous surfaces. Until
examining this section, I had not thought it possible that lava could
have flowed in such uniformly thin sheets over a surface far from
smooth. These little streams closely resemble in composition that great
deluge of lava at Albemarle Island, which likewise must have possessed
a high degree of fluidity.

PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS.

In the lava and in the scoriae of this little crater, I found several
fragments, which, from their angular form, their granular structure,
their freedom from air-cells, their brittle and burnt condition,
closely resembled those fragments of primary rocks which are
occasionally ejected, as at Ascension, from volcanoes. These fragments
consist of glassy albite, much mackled, and with very imperfect
cleavages, mingled with semi-rounded grains, having tarnished, glossy
surfaces, of a steel-blue mineral. The crystals of albite are coated by
a red oxide of iron, appearing like a residual substance; and their
cleavage-planes also are sometimes separated by excessively fine layers
of this oxide, giving to the crystals the appearance of being ruled
like a glass micrometer. There was no quartz. The steel-blue mineral,
which is abundant in the pinnacle, but which disappears in the streams
derived from the pinnacle, has a fused appearance, and rarely presents
even a trace of cleavage; I obtained, however, one measurement, which
proved that it was augite; and in one other fragment, which differed
from the others, in being slightly cellular, and in gradually blending
into the surrounding matrix the small grains of this mineral were
tolerably well crystallised. Although there is so wide a difference in
appearance between the lava of the little streams, and especially of
their red scoriaceous crusts, and one of these angular ejected
fragments, which at first sight might readily be mistaken for syenite,
yet I believe that the lava has originated from the melting and
movement of a mass of rock of absolutely similar composition with the
fragments. Besides the specimen above alluded to, in which we see a
fragment becoming slightly cellular, and blending into the surrounding
matrix, some of the grains of the steel-blue augite also have their
surfaces becoming very finely vesicular, and passing into the nature of
the surrounding paste; other grains are throughout, in an intermediate
condition. The paste seems to consist of the augite more perfectly
fused, or, more probably, merely disturbed in its softened state by the
movement of the mass, and mingled with the oxide of iron and with
finely comminuted, glassy albite. Hence probably it is that the fused
albite, which is abundant in the pinnacle, disappears in the streams.
The albite is in exactly the same state, with the exception of most of
the crystals being smaller in the lava and in the embedded fragments;
but in the fragments they appear to be less abundant: this, however,
would naturally happen from the intumescence of the augitic base, and
its consequent apparent increase in bulk. It is interesting thus to
trace the steps by which a compact granular rock becomes converted into
a vesicular, pseudo-porphyritic lava, and finally into red scoriae. The
structure and composition of the embedded fragments show that they are
parts either of a mass of primary rock which has undergone considerable
change from volcanic action, or more probably of the crust of a body of
cooled and crystallised lava, which has afterwards been broken up and
re-liquified; the crust being less acted on by the renewed heat and
movement.

CONCLUDING REMARKS ON THE TUFF-CRATERS.

These craters, from the peculiarity of the resin-like substance which
enters largely into their composition, from their structure, their size
and number, present the most striking feature in the geology of this
Archipelago. The majority of them form either separate islets, or
promontories attached to the larger islands; and those which now stand
at some little distance from the coast are worn and breached, as if by
the action of the sea. From this general circumstance of their
position, and from the small quantity of ejected ashes in any part of
the Archipelago, I am led to conclude, that the tuff has been chiefly
produced, by the grinding together of fragments of lava within active
craters, communicating with the sea. In the origin and composition of
the tuff, and in the frequent presence of a central lake of brine and
of layers of salt, these craters resemble, though on a gigantic scale,
the “salses,” or hillocks of mud, which are common in some parts of
Italy and in other countries. (D’Aubuisson “Traite de Geognosie” tome 1
page 189. I may remark, that I saw at Terceira, in the Azores, a crater
of tuff or peperino, very similar to these of the Galapagos
Archipelago. From the description given in Freycinet “Voyage,” similar
ones occur at the Sandwich Islands; and probably they are present in
many other places.) Their closer connection, however, in this
Archipelago, with ordinary volcanic action, is shown by the pools of
solidified basalt, with which they are sometimes filled up.

It at first appears very singular, that all the craters formed of tuff
have their southern sides, either quite broken down and wholly removed,
or much lower than the other sides. I saw and received accounts of
twenty-eight of these craters; of these, twelve form separate islets
(These consist of the three Crossman Islets, the largest of which is
600 feet in height; Enchanted Island; Gardner Island (760 feet high);
Champion Island (331 feet high); Enderby Island; Brattle Island; two
islets near Indefatigable Island; and one near James Island. A second
crater near James Island (with a salt lake in its centre) has its
southern side only about twenty feet high, whilst the other parts of
the circumference are about three hundred feet in height.), and now
exist as mere crescents quite open to the south, with occasionally a
few points of rock marking their former circumference: of the remaining
sixteen, some form promontories, and others stand at a little distance
inland from the shore; but all have their southern sides either the
lowest, or quite broken down. Two, however, of the sixteen had their
northern sides also low, whilst their eastern and western sides were
perfect. I did not see, or hear of, a single exception to the rule, of
these craters being broken down or low on the side, which faces a point
of the horizon between S.E. and S.W. This rule does not apply to
craters composed of lava and scoriae. The explanation is simple: at
this Archipelago, the waves from the trade-wind, and the swell
propagated from the distant parts of the open ocean, coincide in
direction (which is not the case in many parts of the Pacific), and
with their united forces attack the southern sides of all the islands;
and consequently the southern slope, even when entirely formed of hard
basaltic rock, is invariably steeper than the northern slope. As the
tuff-craters are composed of a soft material, and as probably all, or
nearly all, have at some period stood immersed in the sea, we need not
wonder that they should invariably exhibit on their exposed sides the
effects of this great denuding power. Judging from the worn condition
of many of these craters, it is probable that some have been entirely
washed away. As there is no reason to suppose, that the craters formed
of scoriae and lava were erupted whilst standing in the sea, we can see
why the rule does not apply to them. At Ascension, it was shown that
the mouths of the craters, which are there all of terrestrial origin,
have been affected by the trade-wind; and this same power might here,
also, aid in making the windward and exposed sides of some of the
craters originally the lowest.

MINERALOGICAL COMPOSITION OF THE ROCKS.

In the northern islands, the basaltic lavas seem generally to contain
more albite than they do in the southern half of the Archipelago; but
almost all the streams contain some. The albite is not unfrequently
associated with olivine. I did not observe in any specimen
distinguishable crystals of hornblende or augite; I except the fused
grains in the ejected fragments, and in the pinnacle of the little
crater, above described. I did not meet with a single specimen of true
trachyte; though some of the paler lavas, when abounding with large
crystals of the harsh and glassy albite, resemble in some degree this
rock; but in every case the basis fuses into a black enamel. Beds of
ashes and far-ejected scoriae, as previously stated, are almost absent;
nor did I see a fragment of obsidian or of pumice. Von Buch believes
that the absence of pumice on Mount Etna is consequent on the feldspar
being of the Labrador variety (“Description des Isles Canaries” page
328.); if the presence of pumice depends on the constitution of the
feldspar, it is remarkable, that it should be absent in this
archipelago, and abundant in the Cordillera of South America, in both
of which regions the feldspar is of the albitic variety. Owing to the
absence of ashes, and the general indecomposable character of the lava
in this Archipelago, the islands are slowly clothed with a poor
vegetation, and the scenery has a desolate and frightful aspect.

ELEVATION OF THE LAND.

Proofs of the rising of the land are scanty and imperfect. At Chatham
Island, I noticed some great blocks of lava, cemented by calcareous
matter, containing recent shells; but they occurred at the height of
only a few feet above high-water mark. One of the officers gave me some
fragments of shells, which he found embedded several hundred feet above
the sea, in the tuff of two craters, distant from each other. It is
possible, that these fragments may have been carried up to their
present height in an eruption of mud; but as, in one instance, they
were associated with broken oyster- shells, almost forming a layer, it
is more probable that the tuff was uplifted with the shells in mass.
The specimens are so imperfect that they can be recognised only as
belonging to recent marine genera. On Charles Island, I observed a line
of great rounded blocks, piled on the summit of a vertical cliff, at
the height of fifteen feet above the line, where the sea now acts
during the heaviest gales. This appeared, at first, good evidence in
favour of the elevation of the land; but it was quite deceptive, for I
afterwards saw on an adjoining part of this same coast, and heard from
eye- witnesses, that wherever a recent stream of lava forms a smooth
inclined plane, entering the sea, the waves during gales have the power
of ROLLING UP ROUNDED blocks to a great height, above the line of their
ordinary action. As the little cliff in the foregoing case is formed by
a stream of lava, which, before being worn back, must have entered the
sea with a gently sloping surface, it is possible or rather it is
probable, that the rounded boulders, now lying on its summit, are
merely the remnants of those which had been ROLLED UP during storms to
their present height.

DIRECTION OF THE FISSURES OF ERUPTION.

The volcanic orifices in this group cannot be considered as
indiscriminately scattered. Three great craters on Albermarle Island
form a well-marked line, extending N.W. by N. and S.E. by S. Narborough
Island, and the great crater on the rectangular projection of Albemarle
Island, form a second parallel line. To the east, Hood’s Island, and
the islands and rocks between it and James Island, form another nearly
parallel line, which, when prolonged, includes Culpepper and Wenman
Islands, lying seventy miles to the north. The other islands lying
further eastward, form a less regular fourth line. Several of these
islands, and the vents on Albemarle Island, are so placed, that they
likewise fall on a set of rudely parallel lines, intersecting the
former lines at right angles; so that the principal craters appear to
lie on the points where two sets of fissures cross each other. The
islands themselves, with the exception of Albemarle Island, are not
elongated in the same direction with the lines on which they stand. The
direction of these islands is nearly the same with that which prevails
in so remarkable a manner in the numerous archipelagoes of the great
Pacific Ocean. Finally, I may remark, that amongst the Galapagos
Islands there is no one dominant vent much higher than all the others,
as may be observed in many volcanic archipelagoes: the highest is the
great mound on the south- western extremity of Albemarle Island, which
exceeds by barely a thousand feet several other neighbouring craters.




CHAPTER VI.
TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.


The sinking of crystals in fluid lava. Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation. Obsidian. Apparent non-separation of the elements of
plutonic rocks. Origin of trap-dikes in the plutonic series.
Distribution of volcanic islands; their prevalence in the great oceans.
They are generally arranged in lines. The central volcanoes of Von Buch
doubtful. Volcanic islands bordering continents. Antiquity of volcanic
islands, and their elevation in mass. Eruptions on parallel lines of
fissure within the same geological period.

ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO
THEIR SPECIFIC GRAVITIES.

One side of Fresh-water Bay, in James Island, is formed by the wreck of
a large crater, mentioned in the last chapter, of which the interior
has been filled up by a pool of basalt, about two hundred feet in
thickness. This basalt is of a grey colour, and contains many crystals
of glassy albite, which become much more numerous in the lower,
scoriaceous part. This is contrary to what might have been expected,
for if the crystals had been originally disseminated in equal numbers,
the greater intumescence of this lower scoriaceous part would have made
them appear fewer in number. Von Buch has described a stream of
obsidian on the Peak of Teneriffe, in which the crystals of feldspar
become more and more numerous, as the depth or thickness increases, so
that near the lower surface of the stream the lava even resembles a
primary rock. (“Description des Isles Canaries” pages 190 and 191.) Von
Buch further states, that M. Dree, in his experiments in melting lava,
found that the crystals of feldspar always tended to precipitate
themselves to the bottom of the crucible. In these cases, I presume
there can be no doubt that the crystals sink from their weight. (In a
mass of molten iron, it is found (“Edinburgh New Philosophical Journal”
volume 24 page 66) that the substances, which have a closer affinity
for oxygen than iron has, rise from the interior of the mass to the
surface. But a similar cause can hardly apply to the separation of the
crystals of these lava-streams. The cooling of the surface of lava
seems, in some cases, to have affected its composition; for Dufrenoy
(“Mem. pour servir” tome 4 page 271) found that the interior parts of a
stream near Naples contained two-thirds of a mineral which was acted on
by acids, whilst the surface consisted chiefly of a mineral
unattackable by acids.) The specific gravity of feldspar varies from
2.4 to 2.58, whilst obsidian seems commonly to be from 2.3 to 2.4; and
in a fluidified state its specific gravity would probably be less,
which would facilitate the sinking of the crystals of feldspar. (I have
taken the specific gravities of the simple minerals from Von Kobell,
one of the latest and best authorities, and of the rocks from various
authorities. Obsidian, according to Phillips, is 2.35; and Jameson says
it never exceeds 2.4; but a specimen from Ascension, weighed by myself,
was 2.42.) At James Island, the crystals of albite, though no doubt of
less weight than the grey basalt, in the parts where compact, might
easily be of greater specific gravity than the scoriaceous mass, formed
of melted lava and bubbles of heated gas.

The sinking of crystals through a viscid substance like molten rock, as
is unequivocally shown to have been the case in the experiments of M.
Dree, is worthy of further consideration, as throwing light on the
separation of the trachytic and basaltic series of lavas. Mr. P. Scrope
has speculated on this subject; but he does not seem to have been aware
of any positive facts, such as those above given; and he has overlooked
one very necessary element, as it appears to me, in the
phenomenon—namely, the existence of either the lighter or heavier
mineral in globules or in crystals. In a substance of imperfect
fluidity, like molten rock, it is hardly credible, that the separate,
infinitely small atoms, whether of feldspar, augite, or of any other
mineral, would have power from their slightly different gravities to
overcome the friction caused by their movement; but if the atoms of any
one of these minerals became, whilst the others remained fluid, united
into crystals or granules, it is easy to perceive that from the
lessened friction, their sinking or floating power would be greatly
increased. On the other hand, if all the minerals became granulated at
the same time, it is scarcely possible, from their mutual resistance,
that any separation could take place. A valuable, practical discovery,
illustrating the effect of the granulation of one element in a fluid
mass, in aiding its separation, has lately been made: when lead
containing a small proportion of silver, is constantly stirred whilst
cooling, it becomes granulated, and the grains of imperfect crystals of
nearly pure lead sink to the bottom, leaving a residue of melted metal
much richer in silver; whereas if the mixture be left undisturbed,
although kept fluid for a length of time, the two metals show no signs
of separating. (A full and interesting account of this discovery, by
Mr. Pattinson, was read before the British Association in September
1838. In some alloys, according to Turner “Chemistry” page 210, the
heaviest metal sinks, and it appears that this takes place whilst both
metals are fluid. Where there is a considerable difference in gravity,
as between iron and the slag formed during the fusion of the ore, we
need not be surprised at the atoms separating, without either substance
being granulated.) The sole use of the stirring seems to be, the
formation of detached granules. The specific gravity of silver is 10.4,
and of lead 11.35: the granulated lead, which sinks, is never
absolutely pure, and the residual fluid metal contains, when richest,
only 1/119 part of silver. As the difference in specific gravity,
caused by the different proportions of the two metals, is so
exceedingly small, the separation is probably aided in a great degree
by the difference in gravity between the lead, when granular though
still hot, and when fluid.

In a body of liquified volcanic rock, left for some time without any
violent disturbance, we might expect, in accordance with the above
facts, that if one of the constituent minerals became aggregated into
crystals or granules, or had been enveloped in this state from some
previously existing mass, such crystals or granules would rise or sink,
according to their specific gravity. Now we have plain evidence of
crystals being embedded in many lavas, whilst the paste or basis has
continued fluid. I need only refer, as instances, to the several,
great, pseudo-porphyritic streams at the Galapagos Islands, and to the
trachytic streams in many parts of the world, in which we find crystals
of feldspar bent and broken by the movement of the surrounding,
semi-fluid matter. Lavas are chiefly composed of three varieties of
feldspar, varying in specific gravity from 2.4 to 2.74; of hornblende
and augite, varying from 3.0 to 3.4; of olivine, varying from 3.3 to
3.4; and lastly, of oxides of iron, with specific gravities from 4.8 to
5.2. Hence crystals of feldspar, enveloped in a mass of liquified, but
not highly vesicular lava, would tend to rise to the upper parts; and
crystals or granules of the other minerals, thus enveloped, would tend
to sink. We ought not, however, to expect any perfect degree of
separation in such viscid materials. Trachyte, which consists chiefly
of feldspar, with some hornblende and oxide of iron, has a specific
gravity of about 2.45; whilst basalt, composed chiefly of augite and
feldspar, often with much iron and olivine, has a gravity of about 3.0.
(Trachyte from Java was found by Von Buch to be 2.47; from Auvergne, by
De la Beche, it was 2.42; from Ascension, by myself, it was 2.42.
Jameson and other authors give to basalt a specific gravity of 3.0; but
specimens from Auvergne were found, by De la Beche, to be only 2.78;
and from the Giant’s Causeway, to be 2.91.) Accordingly we find, that
where both trachytic and basaltic streams have proceeded from the same
orifice, the trachytic streams have generally been first erupted owing,
as we must suppose, to the molten lava of this series having
accumulated in the upper parts of the volcanic focus. This order of
eruption has been observed by Beudant, Scrope, and by other authors;
three instances, also, have been given in this volume. As the later
eruptions, however, from most volcanic mountains, burst through their
basal parts, owing to the increased height and weight of the internal
column of molten rock, we see why, in most cases, only the lower flanks
of the central, trachytic masses, are enveloped by basaltic streams.
The separation of the ingredients of a mass of lava, would, perhaps,
sometimes take place within the body of a volcanic mountain, if lofty
and of great dimensions, instead of within the underground focus; in
which case, trachytic streams might be poured forth, almost
contemporaneously, or at short recurrent intervals, from its summit,
and basaltic streams from its base: this seems to have taken place at
Teneriffe. (Consult Von Buch’s well-known and admirable “Description
Physique” of this island, which might serve as a model of descriptive
geology.) I need only further remark, that from violent disturbances
the separation of the two series, even under otherwise favourable
conditions, would naturally often be prevented, and likewise their
usual order of eruption be inverted. From the high degree of fluidity
of most basaltic lavas, these perhaps, alone, would in many cases reach
the surface.

As we have seen that crystals of feldspar, in the instance described by
Von Buch, sink in obsidian, in accordance with their known greater
specific gravity, we might expect to find in every trachytic district,
where obsidian has flowed as lava, that it had proceeded from the upper
or highest orifices. This, according to Von Buch, holds good in a
remarkable manner both at the Lipari Islands and on the Peak of
Teneriffe; at this latter place obsidian has never flowed from a less
height than 9,200 feet. Obsidian, also, appears to have been erupted
from the loftiest peaks of the Peruvian Cordillera. I will only further
observe, that the specific gravity of quartz varies from 2.6 to 2.8;
and therefore, that when present in a volcanic focus, it would not tend
to sink with the basaltic bases; and this, perhaps, explains the
frequent presence, and the abundance of this mineral, in the lavas of
the trachytic series, as observed in previous parts of this volume.

An objection to the foregoing theory will, perhaps, be drawn from the
plutonic rocks not being separated into two evidently distinct series,
of different specific gravities; although, like the volcanic, they have
been liquified. In answer, it may first be remarked, that we have no
evidence of the atoms of any one of the constituent minerals in the
plutonic series having been aggregated, whilst the others remained
fluid, which we have endeavoured to show is an almost necessary
condition of their separation; on the contrary, the crystals have
generally impressed each other with their forms. (The crystalline paste
of phonolite is frequently penetrated by long needles of hornblende;
from which it appears that the hornblende, though the more fusible
mineral, has crystallised before, or at the same time with a more
refractory substance. Phonolite, as far as my observations serve, in
every instance appears to be an injected rock, like those of the
plutonic series; hence probably, like these latter, it has generally
been cooled without repeated and violent disturbances. Those geologists
who have doubted whether granite could have been formed by igneous
liquefaction, because minerals of different degrees of fusibility
impress each other with their forms, could not have been aware of the
fact of crystallised hornblende penetrating phonolite, a rock
undoubtedly of igneous origin. The viscidity, which it is now known,
that both feldspar and quartz retain at a temperature much below their
points of fusion, easily explains their mutual impressment. Consult on
this subject Mr. Horner’s paper on Bonn “Geolog. Transact.” volume 4
page 439; and “L’Institut” with respect to quartz 1839 page 161.)

In the second place, the perfect tranquillity, under which it is
probable that the plutonic masses, buried at profound depths, have
cooled, would, most likely, be highly unfavourable to the separation of
their constituent minerals; for, if the attractive force, which during
the progressive cooling draws together the molecules of the different
minerals, has power sufficient to keep them together, the friction
between such half-formed crystals or pasty globules would effectually
prevent the heavier ones from sinking, or the lighter ones from rising.
On the other hand, a small amount of disturbance, which would probably
occur in most volcanic foci, and which we have seen does not prevent
the separation of granules of lead from a mixture of molten lead and
silver, or crystals of feldspar from streams of lava, by breaking and
dissolving the less perfectly formed globules, would permit the more
perfect and therefore unbroken crystals, to sink or rise, according to
their specific gravity.

Although in plutonic rocks two distinct species, corresponding to the
trachytic and basaltic series, do not exist, I much suspect that a
certain amount of separation of their constituent parts has often taken
place. I suspect this from having observed how frequently dikes of
greenstone and basalt intersect widely extended formations of granite
and the allied metamorphic rocks. I have never examined a district in
an extensive granitic region without discovering dikes; I may instance
the numerous trap-dikes, in several districts of Brazil, Chile, and
Australia, and at the Cape of Good Hope: many dikes likewise occur in
the great granitic tracts of India, in the north of Europe, and in
other countries. Whence, then, has the greenstone and basalt, forming
these dikes, come? Are we to suppose, like some of the elder
geologists, that a zone of trap is uniformly spread out beneath the
granitic series, which composes, as far as we know, the foundations of
the earth’s crust? Is it not more probable, that these dikes have been
formed by fissures penetrating into partially cooled rocks of the
granitic and metamorphic series, and by their more fluid parts,
consisting chiefly of hornblende, oozing out, and being sucked into
such fissures? At Bahia, in Brazil, in a district composed of gneiss
and primitive greenstone, I saw many dikes, of a dark augitic (for one
crystal certainly was of this mineral) or hornblendic rock, which, as
several appearances clearly proved, either had been formed before the
surrounding mass had become solid, or had together with it been
afterwards thoroughly softened. (Portions of these dikes have been
broken off, and are now surrounded by the primary rocks, with their
laminae conformably winding round them. Dr. Hubbard also (“Silliman’s
Journal” volume 34 page 119), has described an interlacement of
trap-veins in the granite of the White Mountains, which he thinks must
have been formed when both rocks were soft.) On both sides of one of
these dikes, the gneiss was penetrated, to the distance of several
yards, by numerous, curvilinear threads or streaks of dark matter,
which resembled in form clouds of the class called cirrhi- comae; some
few of these threads could be traced to their junction with the dike.
When examining them, I doubted whether such hair-like and curvilinear
veins could have been injected, and I now suspect, that instead of
having been injected from the dike, they were its feeders. If the
foregoing views of the origin of trap-dikes in widely extended granitic
regions far from rocks of any other formation, be admitted as probable,
we may further admit, in the case of a great body of plutonic rock,
being impelled by repeated movements into the axis of a mountain-chain,
that its more liquid constituent parts might drain into deep and unseen
abysses; afterwards, perhaps, to be brought to the surface under the
form, either of injected masses of greenstone and augitic porphyry, or
of basaltic eruptions. (Mr. Phillips “Lardner’s Encyclop.” volume 2
page 115 quotes Von Buch’s statement, that augitic porphyry ranges
parallel to, and is found constantly at the base of, great chains of
mountains. Humboldt, also, has remarked the frequent occurrence of
trap-rock, in a similar position; of which fact I have observed many
examples at the foot of the Chilian Cordillera. The existence of
granite in the axes of great mountain chains is always probable, and I
am tempted to suppose, that the laterally injected masses of augitic
porphyry and of trap, bear nearly the same relation to the granitic
axes which basaltic lavas bear to the central trachytic masses, round
the flanks of which they have so frequently been erupted.) Much of the
difficulty which geologists have experienced when they have compared
the composition of volcanic with plutonic formations, will, I think, be
removed, if we may believe that most plutonic masses have been, to a
certain extent, drained of those comparatively weighty and easily
liquified elements, which compose the trappean and basaltic series of
rocks.

ON THE DISTRIBUTION OF VOLCANIC ISLANDS.

During my investigations on coral-reefs, I had occasion to consult the
works of many voyagers, and I was invariably struck with the fact, that
with rare exceptions, the innumerable islands scattered throughout the
Pacific, Indian, and Atlantic Oceans, were composed either of volcanic,
or of modern coral-rocks. It would be tedious to give a long catalogue
of all the volcanic islands; but the exceptions which I have found are
easily enumerated: in the Atlantic, we have St. Paul’s Rock, described
in this volume, and the Falkland Islands, composed of quartz and
clay-slate; but these latter islands are of considerable size, and lie
not very far from the South American coast (Judging from Forster’s
imperfect observation, perhaps Georgia is not volcanic. Dr. Allan is my
informant with regard to the Seychelles. I do not know of what
formation Rodriguez, in the Indian Ocean, is composed.): in the Indian
Ocean, the Seychelles (situated in a line prolonged from Madagascar)
consist of granite and quartz: in the Pacific Ocean, New Caledonia, an
island of large size, belongs (as far as is known) to the primary
class. New Zealand, which contains much volcanic rock and some active
volcanoes, from its size cannot be classed with the small islands,
which we are now considering. The presence of a small quantity of
non-volcanic rock, as of clay-slate on three of the Azores (This is
stated on the authority of Count V. de Bedemar, with respect to Flores
and Graciosa (Charlsworth “Magazine of Nat. Hist.” volume 1 page 557).
St. Maria has no volcanic rock, according to Captain Boyd (Von Buch
“Descript.” page 365). Chatham Island has been described by Dr.
Dieffenbach in the “Geographical Journal” 1841 page 201. As yet we have
received only imperfect notices on Kerguelen Land, from the Antarctic
Expedition.), or of tertiary limestone at Madeira, or of clay-slate at
Chatham Island in the Pacific, or of lignite at Kerguelen Land, ought
not to exclude such islands or archipelagoes, if formed chiefly of
erupted matter, from the volcanic class.

The composition of the numerous islands scattered through the great
oceans being with such rare exceptions volcanic, is evidently an
extension of that law, and the effect of those same causes, whether
chemical or mechanical, from which it results, that a vast majority of
the volcanoes now in action stand either as islands in the sea, or near
its shores. This fact of the ocean-islands being so generally volcanic
is also interesting in relation to the nature of the mountain-chains on
our continents, which are comparatively seldom volcanic; and yet we are
led to suppose that where our continents now stand an ocean once
extended. Do volcanic eruptions, we may ask, reach the surface more
readily through fissures formed during the first stages of the
conversion of the bed of the ocean into a tract of land?

Looking at the charts of the numerous volcanic archipelagoes, we see
that the islands are generally arranged either in single, double, or
triple rows, in lines which are frequently curved in a slight degree.
(Professors William and Henry Darwin Rogers have lately insisted much,
in a memoir read before the American Association, on the regularly
curved lines of elevation in parts of the Appalachian range.) Each
separate island is either rounded, or more generally elongated in the
same direction with the group in which it stands, but sometimes
transversely to it. Some of the groups which are not much elongated
present little symmetry in their forms; M. Virlet (“Bulletin de la Soc.
Geolog.” tome 3 page 110.) states that this is the case with the
Grecian Archipelago: in such groups I suspect (for I am aware how easy
it is to deceive oneself on these points), that the vents are generally
arranged on one line, or on a set of short parallel lines, intersecting
at nearly right angles another line, or set of lines. The Galapagos
Archipelago offers an example of this structure, for most of the
islands and the chief orifices on the largest island are so grouped as
to fall on a set of lines ranging about N.W. by N., and on another set
ranging about W.S.W.: in the Canary Archipelago we have a simpler
structure of the same kind: in the Cape de Verde group, which appears
to be the least symmetrical of any oceanic volcanic archipelago, a N.W.
and S.E. line formed by several islands, if prolonged, would intersect
at right angles a curved line, on which the remaining islands are
placed.

Von Buch (“Description des Isles Canaries” page 324.) has classed all
volcanoes under two heads, namely, CENTRAL VOLCANOES, round which
numerous eruptions have taken place on all sides, in a manner almost
regular, and VOLCANIC CHAINS. In the examples given of the first class,
as far as position is concerned, I can see no grounds for their being
called “central;” and the evidence of any difference in mineralogical
nature between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No
doubt some one island in most small volcanic archipelagoes is apt to be
considerably higher than the others; and in a similar manner, whatever
the cause may be, that on the same island one vent is generally higher
than all the others. Von Buch does not include in his class of volcanic
chains small archipelagoes, in which the islands are admitted by him,
as at the Azores, to be arranged in lines; but when viewing on a map of
the world how perfect a series exists from a few volcanic islands
placed in a row to a train of linear archipelagoes following each other
in a straight line, and so on to a great wall like the Cordillera of
America, it is difficult to believe that there exists any essential
difference between short and long volcanic chains. Von Buch (Idem page
393.) states that his volcanic chains surmount, or are closely
connected with, mountain-ranges of primary formation: but if trains of
linear archipelagoes are, in the course of time, by the long- continued
action of the elevatory and volcanic forces, converted into
mountain-ranges, it would naturally result that the inferior primary
rocks would often be uplifted and brought into view.

Some authors have remarked that volcanic islands occur scattered,
though at very unequal distances, along the shores of the great
continents, as if in some measure connected with them. In the case of
Juan Fernandez, situated 330 miles from the coast of Chile, there was
undoubtedly a connection between the volcanic forces acting under this
island and under the continent, as was shown during the earthquake of
1835. The islands, moreover, of some of the small volcanic groups which
thus border continents, are placed in lines, related to those along
which the adjoining shores of the continents trend; I may instance the
lines of intersection at the Galapagos, and at the Cape de Verde
Archipelagoes, and the best marked line of the Canary Islands. If these
facts be not merely accidental, we see that many scattered volcanic
islands and small groups are related not only by proximity, but in the
direction of the fissures of eruption to the neighbouring continents—a
relation, which Von Buch considers, characteristic of his great
volcanic chains.

In volcanic archipelagoes, the orifices are seldom in activity on more
than one island at a time; and the greater eruptions usually recur only
after long intervals. Observing the number of craters, that are usually
found on each island of a group, and the vast amount of matter which
has been erupted from them, one is led to attribute a high antiquity
even to those groups, which appear, like the Galapagos, to be of
comparatively recent origin. This conclusion accords with the
prodigious amount of degradation, by the slow action of the sea, which
their originally sloping coasts must have suffered, when they are worn
back, as is so often the case, into grand precipices. We ought not,
however, to suppose, in hardly any instance, that the whole body of
matter, forming a volcanic island, has been erupted at the level, on
which it now stands: the number of dikes, which seem invariably to
intersect the interior parts of every volcano, show, on the principles
explained by M. Elie de Beaumont, that the whole mass has been uplifted
and fissured. A connection, moreover, between volcanic eruptions and
contemporaneous elevations in mass has, I think, been shown to exist in
my work on Coral-Reefs, both from the frequent presence of upraised
organic remains, and from the structure of the accompanying
coral-reefs. (A similar conclusion is forced on us, by the phenomena,
which accompanied the earthquake of 1835, at Concepcion, and which are
detailed in my paper (volume 5 page 601) in the “Geological
Transactions.”) Finally, I may remark, that in the same Archipelago,
eruptions have taken place within the historical period on more than
one of the parallel lines of fissure: thus, at the Galapagos
Archipelago, eruptions have taken place from a vent on Narborough
Island, and from one on Albemarle Island, which vents do not fall on
the same line; at the Canary Islands, eruptions have taken place in
Teneriffe and Lanzarote; and at the Azores, on the three parallel lines
of Pico, St. Jorge, and Terceira. Believing that a mountain-axis
differs essentially from a volcano, only in plutonic rocks having been
injected, instead of volcanic matter having been ejected, this appears
to me an interesting circumstance; for we may infer from it as
probable, that in the elevation of a mountain-chain, two or more of the
parallel lines forming it may be upraised and injected within the same
geological period.




CHAPTER VII.
AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.


New South Wales. Sandstone formation. Embedded pseudo-fragments of
shale. Stratification. Current-cleavage. Great valleys. Van Diemen’s
Land. Palaeozoic formation. Newer formation with volcanic rocks.
Travertin with leaves of extinct plants. Elevation of the land. New
Zealand. King George’s Sound. Superficial ferruginous beds. Superficial
calcareous deposits, with casts of branches. Their origin from drifted
particles of shells and corals. Their extent. Cape of Good Hope.
Junction of the granite and clay-slate. Sandstone formation.

The “Beagle,” in her homeward voyage, touched at New Zealand,
Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to
confine the Third Part of these Geological Observations to South
America, I will here briefly describe all that I observed at these
places worthy of the attention of geologists.

NEW SOUTH WALES.

My opportunities of observation consisted of a ride of ninety
geographical miles to Bathurst, in a W.N.W. direction from Sydney. The
first thirty miles from the coast passes over a sandstone country,
broken up in many places by trap-rocks, and separated by a bold
escarpment overhanging the river Nepean, from the great sandstone
platform of the Blue Mountains. This upper platform is 1,000 feet high
at the edge of the escarpment, and rises in a distance of twenty-five
miles to between three and four thousand feet above the level of the
sea. At this distance the road descends to a country rather less
elevated, and composed in chief part of primary rocks. There is much
granite, in one part passing into a red porphyry with octagonal
crystals of quartz, and intersected in some places by trap-dikes. Near
the Downs of Bathurst I passed over much pale-brown, glossy clay-slate,
with the shattered laminae running north and south; I mention this
fact, because Captain King informs me that, in the country a hundred
miles southward, near Lake George, the mica-slate ranges so invariably
north and south that the inhabitants take advantage of it in finding
their way through the forests.

The sandstone of the Blue Mountains is at least 1,200 feet thick, and
in some parts is apparently of greater thickness; it consists of small
grains of quartz, cemented by white earthy matter, and it abounds with
ferruginous veins. The lower beds sometimes alternate with shales and
coal: at Wolgan I found in carbonaceous shale leaves of the
Glossopteris Brownii, a fern which so frequently accompanies the coal
of Australia. The sandstone contains pebbles of quartz; and these
generally increase in number and size (seldom, however, exceeding an
inch or two in diameter) in the upper beds: I observed a similar
circumstance in the grand sandstone formation at the Cape of Good Hope.
On the South American coast, where tertiary and supra- tertiary beds
have been extensively elevated, I repeatedly noticed that the uppermost
beds were formed of coarser materials than the lower: this appears to
indicate that, as the sea became shallower, the force of the waves or
currents increased. On the lower platform, however, between the Blue
Mountains and the coast, I observed that the upper beds of the
sandstone frequently passed into argillaceous shale,—the effect,
probably, of this lower space having been protected from strong
currents during its elevation. The sandstone of the Blue Mountains
evidently having been of mechanical origin, and not having suffered any
metamorphic action, I was surprised at observing that, in some
specimens, nearly all the grains of quartz were so perfectly
crystallised with brilliant facets that they evidently had not in their
PRESENT form been aggregated in any previously existing rock. (I have
lately seen, in a paper by Smith (the father of English geologists), in
the “Magazine of Natural History,” that the grains of quartz in the
millstone grit of England are often crystallised. Sir David Brewster,
in a paper read before the British Association, 1840, states, that in
old decomposed glass, the silex and metals separate into concentric
rings, and that the silex regains its crystalline structure, as is
shown by its action on light.) It is difficult to imagine how these
crystals could have been formed; one can hardly believe that they were
separately precipitated in their present crystallised state. Is it
possible that rounded grains of quartz may have been acted on by a
fluid corroding their surfaces, and depositing on them fresh silica? I
may remark that, in the sandstone formation of the Cape of Good Hope,
it is evident that silica has been profusely deposited from aqueous
solution.

In several parts of the sandstone I noticed patches of shale which
might at the first glance have been mistaken for extraneous fragments;
their horizontal laminae, however, being parallel with those of the
sandstone, showed that they were the remnants of thin, continuous beds.
One such fragment (probably the section of a long narrow strip) seen in
the face of a cliff, was of greater vertical thickness than breadth,
which proves that this bed of shale must have been in some slight
degree consolidated, after having been deposited, and before being worn
away by the currents. Each patch of the shale shows, also, how slowly
many of the successive layers of sandstone were deposited. These
pseudo-fragments of shale will perhaps explain, in some cases, the
origin of apparently extraneous fragments in crystalline metamorphic
rocks. I mention this, because I found near Rio de Janeiro a
well-defined angular fragment, seven yards long by two yards in
breadth, of gneiss containing garnets and mica in layers, enclosed in
the ordinary, stratified, porphyritic gneiss of the country. The
laminae of the fragment and of the surrounding matrix ran in exactly
the same direction, but they dipped at different angles. I do not wish
to affirm that this singular fragment (a solitary case, as far as I
know) was originally deposited in a layer, like the shale in the Blue
Mountains, between the strata of the porphyritic gneiss, before they
were metamorphosed; but there is sufficient analogy between the two
cases to render such an explanation possible.

STRATIFICATION OF THE ESCARPMENT.

The strata of the Blue Mountains appear to the eye horizontal; but they
probably have a similar inclination with the surface of the platform,
which slopes from the west towards the escarpment over the Nepean, at
an angle of one degree, or of one hundred feet in a mile. (This is
stated on the authority of Sir T. Mitchell in “Travels” volume 2 page
357.) The strata of the escarpment dip almost conformably with its
steeply inclined face, and with so much regularity, that they appear as
if thrown into their present position; but on a more careful
examination, they are seen to thicken and to thin out, and in the upper
part to be succeeded and almost capped by horizontal beds. These
appearances render it probable, that we here see an original
escarpment, not formed by the sea having eaten back into the strata,
but by the strata having originally extended only thus far. Those who
have been in the habit of examining accurate charts of sea-coasts,
where sediment is accumulating, will be aware, that the surfaces of the
banks thus formed, generally slope from the coast very gently towards a
certain line in the offing, beyond which the depth in most cases
suddenly becomes great. I may instance the great banks of sediment
within the West Indian Archipelago (I have described these very curious
banks in the Appendix to my volume on the structure of Coral-Reefs. I
have ascertained the inclination of the edges of the banks, from
information given me by Captain B. Allen, one of the surveyors, and by
carefully measuring the horizontal distances between the last sounding
on the bank and the first in the deep water. Widely extended banks in
all parts of the West Indies have the same general form of surface.),
which terminate in submarine slopes, inclined at angles of between
thirty and forty degrees, and sometimes even at more than forty
degrees: every one knows how steep such a slope would appear on the
land. Banks of this nature, if uplifted, would probably have nearly the
same external form as the platform of the Blue Mountains, where it
abruptly terminates over the Nepean.

CURRENT-CLEAVAGE.

The strata of sandstone in the low coast country, and likewise on the
Blue Mountains, are often divided by cross or current laminae, which
dip in different directions, and frequently at an angle of forty-five
degrees. Most authors have attributed these cross layers to successive
small accumulations on an inclined surface; but from a careful
examination in some parts of the New Red Sandstone of England, I
believe that such layers generally form parts of a series of curves,
like gigantic tidal ripples, the tops of which have since been cut off,
either by nearly horizontal layers, or by another set of great ripples,
the folds of which do not exactly coincide with those below them. It is
well-known to surveyors that mud and sand are disturbed during storms
at considerable depths, at least from three hundred to four hundred and
fifty feet (See Martin White on “Soundings in the British Channel”
pages 4 and 166.), so that the nature of the bottom even becomes
temporarily changed; the bottom, also, at a depth between sixty and
seventy feet, has been observed to be broadly rippled. (M. Siau on the
“Action of Waves” “Edin. New Phil. Journ.” volume 31 page 245.) One
may, therefore, be allowed to suspect, from the appearance just
mentioned in the New Red Sandstone, that at greater depths, the bed of
the ocean is heaped up during gales into great ripple-like furrows and
depressions, which are afterwards cut off by the currents during more
tranquil weather, and again furrowed during gales.

VALLEYS IN THE SANDSTONE PLATFORMS.

The grand valleys, by which the Blue Mountains and the other sandstone
platforms of this part of Australia are penetrated, and which long
offered an insuperable obstacle to the attempts of the most
enterprising colonist to reach the interior country, form the most
striking feature in the geology of New South Wales. They are of grand
dimensions, and are bordered by continuous links of lofty cliffs. It is
not easy to conceive a more magnificent spectacle, than is presented to
a person walking on the summit- plains, when without any notice he
arrives at the brink of one of these cliffs, which are so
perpendicular, that he can strike with a stone (as I have tried) the
trees growing, at the depth of between one thousand and one thousand
five hundred feet below him; on both hands he sees headland beyond
headland of the receding line of cliff, and on the opposite side of the
valley, often at the distance of several miles, he beholds another line
rising up to the same height with that on which he stands, and formed
of the same horizontal strata of pale sandstone. The bottoms of these
valleys are moderately level, and the fall of the rivers flowing in
them, according to Sir T. Mitchell, is gentle. The main valleys often
send into the platform great baylike arms, which expand at their upper
ends; and on the other hand, the platform often sends promontories into
the valley, and even leaves in them great, almost insulated, masses. So
continuous are the bounding lines of cliff, that to descend into some
of these valleys, it is necessary to go round twenty miles; and into
others, the surveyors have only lately penetrated, and the colonists
have not yet been able to drive in their cattle. But the most
remarkable point of structure in these valleys, is, that although
several miles wide in their upper parts, they generally contract
towards their mouths to such a degree as to become impassable. The
Surveyor-General, Sir T. Mitchell, in vain endeavoured, first on foot
and then by crawling between the great fallen fragments of sandstone,
to ascend through the gorge by which the river Grose joins the Nepean
(“Travels in Australia” volume 1 page 154.—I must express my obligation
to Sir T. Mitchell for several interesting personal communications on
the subject of these great valleys of New South Wales.); yet the valley
of the Grose in its upper part, as I saw, forms a magnificent basin
some miles in width, and is on all sides surrounded by cliffs, the
summits of which are believed to be nowhere less than 3,000 feet above
the level of the sea. When cattle are driven into the valley of the
Wolgan by a path (which I descended) partly cut by the colonists, they
cannot escape; for this valley is in every other part surrounded by
perpendicular cliffs, and eight miles lower down, it contracts, from an
average width of half a mile, to a mere chasm impassable to man or
beast. Sir T. Mitchell states, that the great valley of the Cox river
with all its branches contracts, where it unites with the Nepean, into
a gorge 2,200 yards wide, and about one thousand feet in depth. (Idem
volume 2 page 358.) Other similar cases might have been added.

The first impression, from seeing the correspondence of the horizontal
strata, on each side of these valleys and great amphitheatre-like
depressions, is that they have been in chief part hollowed out, like
other valleys, by aqueous erosion; but when one reflects on the
enormous amount of stone, which on this view must have been removed, in
most of the above cases through mere gorges or chasms, one is led to
ask whether these spaces may not have subsided. But considering the
form of the irregularly branching valleys, and of the narrow
promontories, projecting into them from the platforms, we are compelled
to abandon this notion. To attribute these hollows to alluvial action,
would be preposterous; nor does the drainage from the summit-level
always fall, as I remarked near the Weatherboard, into the head of
these valleys, but into one side of their bay-like recesses. Some of
the inhabitants remarked to me, that they never viewed one of these
baylike recesses, with the headlands receding on both hands, without
being struck with their resemblance to a bold sea-coast. This is
certainly the case; moreover, the numerous fine harbours, with their
widely branching arms, on the present coast of New South Wales, which
are generally connected with the sea by a narrow mouth, from one mile
to a quarter of a mile in width, passing through the sandstone
coast-cliffs, present a likeness, though on a miniature scale, to the
great valleys of the interior. But then immediately occurs the
startling difficulty, why has the sea worn out these great, though
circumscribed, depressions on a wide platform, and left mere gorges,
through which the whole vast amount of triturated matter must have been
carried away? The only light I can throw on this enigma, is by showing
that banks appear to be forming in some seas of the most irregular
forms, and that the sides of such banks are so steep (as before stated)
that a comparatively small amount of subsequent erosion would form them
into cliffs: that the waves have power to form high and precipitous
cliffs, even in landlocked harbours, I have observed in many parts of
South America. In the Red Sea, banks with an extremely irregular
outline and composed of sediment, are penetrated by the most singularly
shaped creeks with narrow mouths: this is likewise the case, though on
a larger scale, with the Bahama Banks. Such banks, I have been led to
suppose, have been formed by currents heaping sediment on an irregular
bottom. (See the “Appendix” to the Part on Coral-Reefs. The fact of the
sea heaping up mud round a submarine nucleus, is worthy of the notice
of geologists: for outlyers of the same composition with the coast
banks are thus formed; and these, if upheaved and worn into cliffs,
would naturally be thought to have been once connected together.) That
in some cases, the sea, instead of spreading out sediment in a uniform
sheet, heaps it round submarine rocks and islands, it is hardly
possible to doubt, after having examined the charts of the West Indies.
To apply these ideas to the sandstone platforms of New South Wales, I
imagine that the strata might have been heaped on an irregular bottom
by the action of strong currents, and of the undulations of an open
sea; and that the valley-like spaces thus left unfilled might, during a
slow elevation of the land, have had their steeply sloping flanks worn
into cliffs; the worn-down sandstone being removed, either at the time
when the narrow gorges were cut by the retreating sea, or subsequently
by alluvial action.

VAN DIEMEN’S LAND.

The southern part of this island is mainly formed of mountains of
greenstone, which often assumes a syenitic character, and contains much
hypersthene. These mountains, in their lower half, are generally
encased by strata containing numerous small corals and some shells.
These shells have been examined by Mr. G.B. Sowerby, and have been
described by him: they consist of two species of Producta, and of six
of Spirifera; two of these, namely, P. rugata and S. rotundata,
resemble, as far as their imperfect condition allows of comparison,
British mountain-limestone shells. Mr. Lonsdale has had the kindness to
examine the corals; they consist of six undescribed species, belonging
to three genera. Species of these genera occur in the Silurian,
Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks,
that all these fossils have undoubtedly a Palaeozoic character, and
that probably they correspond in age to a division of the system above
the Silurian formations.

The strata containing these remains are singular from the extreme
variability of their mineralogical composition. Every intermediate form
is present, between flinty-slate, clay-slate passing into grey wacke,
pure limestone, sandstone, and porcellanic rock; and some of the beds
can only be described as composed of a siliceo-calcareo-clay-slate. The
formation, as far as I could judge, is at least a thousand feet in
thickness: the upper few hundred feet usually consist of a siliceous
sandstone, containing pebbles and no organic remains; the inferior
strata, of which a pale flinty slate is perhaps the most abundant, are
the most variable; and these chiefly abound with the remains. Between
two beds of hard crystalline limestone, near Newtown, a layer of white
soft calcareous matter is quarried, and is used for whitewashing
houses. From information given to me by Mr. Frankland, the
Surveyor-General, it appears that this Palaeozoic formation is found in
different parts of the whole island; from the same authority, I may
add, that on the north-eastern coast and in Bass’ Straits primary rocks
extensively occur.

The shores of Storm Bay are skirted, to the height of a few hundred
feet, by strata of sandstone, containing pebbles of the formation just
described, with its characteristic fossils, and therefore belonging to
a subsequent age. These strata of sandstone often pass into shale, and
alternate with layers of impure coal; they have in many places been
violently disturbed. Near Hobart Town, I observed one dike, nearly a
hundred yards in width, on one side of which the strata were tilted at
an angle of 60 degrees, and on the other they were in some parts
vertical, and had been altered by the effects of the heat. On the west
side of Storm Bay, I found these strata capped by streams of basaltic
lava with olivine; and close by there was a mass of brecciated scoriae,
containing pebbles of lava, which probably marks the place of an
ancient submarine crater. Two of these streams of basalt were separated
from each other by a layer of argillaceous wacke, which could be traced
passing into partially altered scoriae. The wacke contained numerous
rounded grains of a soft, grass-green mineral, with a waxy lustre, and
translucent on its edges: under the blowpipe it instantly blackened,
and the points fused into a strongly magnetic, black enamel. In these
characters, it resembles those masses of decomposed olivine, described
at St. Jago in the Cape de Verde group; and I should have thought that
it had thus originated, had I not found a similar substance, in
cylindrical threads, within the cells of the vesicular basalt,—a state
under which olivine never appears; this substance, I believe, would be
classed as bole by mineralogists. (Chlorophaeite, described by Dr.
MacCulloch (“Western Islands” volume 1 page 504) as occurring in a
basaltic amygdaloid, differs from this substance, in remaining
unchanged before the blowpipe, and in blackening from exposure to the
air. May we suppose that olivine, in undergoing the remarkable change
described at St. Jago, passes through several states?)

TRAVERTIN WITH EXTINCT PLANTS.

Behind Hobart Town there is a small quarry of a hard travertin, the
lower strata of which abound with distinct impressions of leaves. Mr.
Robert Brown has had the kindness to look at my specimens, and he
informed me that there are four or five kinds, none of which he
recognises as belonging to existing species. The most remarkable leaf
is palmate, like that of a fan- palm, and no plant having leaves of
this structure has hitherto been discovered in Van Diemen’s Land. The
other leaves do not resemble the most usual form of the Eucalyptus (of
which tribe the existing forests are chiefly composed), nor do they
resemble that class of exceptions to the common form of the leaves of
the Eucalyptus, which occur in this island. The travertin containing
this remnant of a lost vegetation, is of a pale yellow colour, hard,
and in parts even crystalline; but not compact, and is everywhere
penetrated by minute, tortuous, cylindrical pores. It contains a very
few pebbles of quartz, and occasionally layers of chalcedonic nodules,
like those of chert in our Greensand. From the pureness of this
calcareous rock, it has been searched for in other places, but has
never been found. From this circumstance, and from the character of the
deposit, it was probably formed by a calcareous spring entering a small
pool or narrow creek. The strata have subsequently been tilted and
fissured; and the surface has been covered by a singular mass, with
which, also, a large fissure has been filled up, formed of balls of
trap embedded in a mixture of wacke and a white, earthy,
alumino-calcareous substance. Hence it would appear, as if a volcanic
eruption had taken place on the borders of the pool, in which the
calcareous matter was depositing, and had broken it up and drained it.

ELEVATION OF THE LAND.

Both the eastern and western shores of the bay, in the neighbourhood of
Hobart Town, are in most parts covered to the height of thirty feet
above the level of high-water mark, with broken shells, mingled with
pebbles. The colonists attribute these shells to the aborigines having
carried them up for food: undoubtedly, there are many large mounds, as
was pointed out to me by Mr. Frankland, which have been thus formed;
but I think from the numbers of the shells, from their frequent small
size, from the manner in which they are thinly scattered, and from some
appearances in the form of the land, that we must attribute the
presence of the greater number to a small elevation of the land. On the
shore of Ralph Bay (opening into Storm Bay) I observed a continuous
beach about fifteen feet above high-water mark, clothed with
vegetation, and by digging into it, pebbles encrusted with Serpulae
were found: along the banks, also, of the river Derwent, I found a bed
of broken sea-shells above the surface of the river, and at a point
where the water is now much too fresh for sea-shells to live; but in
both these cases, it is just possible, that before certain spits of
sand and banks of mud in Storm Bay were accumulated, the tides might
have risen to the height where we now find the shells. ( It would
appear that some changes are now in progress in Ralph Bay, for I was
assured by an intelligent farmer, that oysters were formerly abundant
in it, but that about the year 1834 they had, without any apparent
cause, disappeared. In the “Transactions of the Maryland Academy”
volume 1 part 1 page 28 there is an account by Mr. Ducatel of vast beds
of oysters and clams having been destroyed by the gradual filling up of
the shallow lagoons and channels, on the shores of the southern United
States. At Chiloe, in South America, I heard of a similar loss,
sustained by the inhabitants, in the disappearance from one part of the
coast of an edible species of Ascidia.)

Evidence more or less distinct of a change of level between the land
and water, has been detected on almost all the land on this side of the
globe. Captain Grey, and other travellers, have found in Southern
Australia upraised shells, belonging either to the recent, or to a late
tertiary period. The French naturalists in Baudin’s expedition, found
shells similarly circumstanced on the S.W. coast of Australia. The Rev.
W.B. Clarke finds proofs of the elevation of the land, to the amount of
400 feet, at the Cape of Good Hope. (“Proceedings of the Geological
Society” volume 3 page 420.) In the neighbourhood of the Bay of Islands
in New Zealand, I observed that the shores were scattered to some
height, as at Van Diemen’s Land, with sea-shells, which the colonists
attribute to the natives. (I will here give a catalogue of the rocks
which I met with near the Bay of Islands, in New Zealand:—1st, Much
basaltic lava, and scoriform rocks, forming distinct craters;—2nd, A
castellated hill of horizontal strata of flesh-coloured limestone,
showing when fractured distinct crystalline facets: the rain has acted
on this rock in a remarkable manner, corroding its surface into a
miniature model of an Alpine country: I observed here layers of chert
and clay ironstone; and in the bed of a stream, pebbles of
clay-slate;—3rd, The shores of the Bay of Islands are formed of a
feldspathic rock, of a bluish-grey colour, often much decomposed, with
an angular fracture, and crossed by numerous ferruginous seams, but
without any distinct stratification or cleavage. Some varieties are
highly crystalline, and would at once be pronounced to be trap; others
strikingly resembled clay-slate, slightly altered by heat: I was unable
to form any decided opinion on this formation.) Whatever may have been
the origin of these shells, I cannot doubt, after having seen a section
of the valley of the Thames River (37 degrees S.), drawn by the Rev. W.
Williams, that the land has been there elevated: on the opposite sides
of this great valley, three step-like terraces, composed of an enormous
accumulation of rounded pebbles, exactly correspond with each other:
the escarpment of each terrace is about fifty feet in height. No one
after having examined the terraces in the valleys on the western shores
of South America, which are strewed with sea-shells, and have been
formed during intervals of rest in the slow elevation of the land,
could doubt that the New Zealand terraces have been similarly formed. I
may add, that Dr. Dieffenbach, in his description of the Chatham
Islands (“Geographical Journal” volume 11 pages 202, 205.) (S.W. of New
Zealand), states that it is manifest “that the sea has left many places
bare which were once covered by its waters.”

KING GEORGE’S SOUND.

This settlement is situated at the south-western angle of the
Australian continent: the whole country is granitic, with the
constituent minerals sometimes obscurely arranged in straight or curved
laminae. In these cases, the rock would be called by Humboldt,
gneiss-granite, and it is remarkable that the form of the bare conical
hills, appearing to be composed of great folding layers, strikingly
resembles, on a small scale, those composed of gneiss-granite at Rio de
Janeiro, and those described by Humboldt at Venezuela. These plutonic
rocks are, in many places, intersected by trappean-dikes; in one place,
I found ten parallel dikes ranging in an E. and W. line; and not far
off another set of eight dikes, composed of a different variety of
trap, ranging at right angles to the former ones. I have observed in
several primary districts, the occurrence of systems of dikes parallel
and close to each other.

SUPERFICIAL FERRUGINOUS BEDS.

The lower parts of the country are everywhere covered by a bed,
following the inequalities of the surface, of a honeycombed sandstone,
abounding with oxides of iron. Beds of nearly similar composition are
common, I believe, along the whole western coast of Australia, and on
many of the East Indian islands. At the Cape of Good Hope, at the base
of the mountains formed of granite and capped with sandstone, the
ground is everywhere coated either by a fine-grained, rubbly,
ochraceous mass, like that at King George’s Sound, or by a coarser
sandstone with fragments of quartz, and rendered hard and heavy by an
abundance of the hydrate of iron, which presents, when freshly broken,
a metallic lustre. Both these varieties have a very irregular texture,
including spaces either rounded or angular, full of loose sand: from
this cause the surface is always honeycombed. The oxide of iron is most
abundant on the edges of the cavities, where alone it affords a
metallic fracture. In these formations, as well as in many true
sedimentary deposits, it is evident that iron tends to become
aggregated, either in the form of a shell, or of a network. The origin
of these superficial beds, though sufficiently obscure, seems to be due
to alluvial action on detritus abounding with iron.

SUPERFICIAL CALCAREOUS DEPOSIT.

A calcareous deposit on the summit of Bald Head, containing branched
bodies, supposed by some authors to have been corals, has been
celebrated by the descriptions of many distinguished voyagers. (I
visited this hill, in company with Captain Fitzroy, and we came to a
similar conclusion regarding these branching bodies.) It folds round
and conceals irregular hummocks of granite, at the height of 600 feet
above the level of the sea. It varies much in thickness; where
stratified, the beds are often inclined at high angles, even as much as
at thirty degrees, and they dip in all directions. These beds are
sometimes crossed by oblique and even-sided laminae. The deposit
consists either of a fine, white calcareous powder, in which not a
trace of structure can be discovered, or of exceedingly minute, rounded
grains, of brown, yellowish, and purplish colours; both varieties being
generally, but not always, mixed with small particles of quartz, and
being cemented into a more or less perfect stone. The rounded
calcareous grains, when heated in a slight degree, instantly lose their
colours; in this and in every other respect, closely resembling those
minute, equal- sized particles of shells and corals, which at St.
Helena have been drifted up the side of the mountains, and have thus
been winnowed of all coarser fragments. I cannot doubt that the
coloured calcareous particles here have had a similar origin. The
impalpable powder has probably been derived from the decay of the
rounded particles; this certainly is possible, for on the coast of
Peru, I have traced LARGE UNBROKEN shells gradually falling into a
substance as fine as powdered chalk. Both of the above-mentioned
varieties of calcareous sandstone frequently alternate with, and blend
into, thin layers of a hard substalagmitic rock, which, even when the
stone on each side contains particles of quartz, is entirely free from
them (I adopt this term from Lieutenant Nelson’s excellent paper on the
Bermuda Islands “Geolog. Trans.” volume 5 page 106, for the hard,
compact, cream- or brown- coloured stone, without any crystalline
structure, which so often accompanies superficial calcareous
accumulations. I have observed such superficial beds, coated with
substalagmitic rock, at the Cape of Good Hope, in several parts of
Chile, and over wide spaces in La Plata and Patagonia. Some of these
beds have been formed from decayed shells, but the origin of the
greater number is sufficiently obscure. The causes which determine
water to dissolve lime, and then soon to redeposit it, are not, I
think, known. The surface of the substalagmitic layers appears always
to be corroded by the rain-water. As all the above-mentioned countries
have a long dry season, compared with the rainy one, I should have
thought that the presence of the substalagmitic was connected with the
climate, had not Lieutenant Nelson found this substance forming under
sea-water. Disintegrated shell seems to be extremely soluble; of which
I found good evidence, in a curious rock at Coquimbo in Chile, which
consisted of small, pellucid, empty husks, cemented together. A series
of specimens clearly showed that these husks had originally contained
small rounded particles of shells, which had been enveloped and
cemented together by calcareous matter (as often happens on
sea-beaches), and which subsequently had decayed, and been dissolved by
water, that must have penetrated through the calcareous husks, without
corroding them,—of which processes every stage could be seen.): hence
we must suppose that these layers, as well as certain vein- like
masses, have been formed by rain dissolving the calcareous matter and
re-precipitating it, as has happened at St. Helena. Each layer probably
marks a fresh surface, when the, now firmly cemented, particles existed
as loose sand. These layers are sometimes brecciated and re-cemented,
as if they had been broken by the slipping of the sand when soft. I did
not find a single fragment of a sea-shell; but bleached shells of the
Helix melo, an existing land species, abound in all the strata; and I
likewise found another Helix, and the case of an Oniscus.

The branches are absolutely undistinguishable in shape from the broken
and upright stumps of a thicket; their roots are often uncovered, and
are seen to diverge on all sides; here and there a branch lies
prostrate. The branches generally consist of the sandstone, rather
firmer than the surrounding matter, with the central parts filled,
either with friable, calcareous matter, or with a substalagmitic
variety; this central part is also frequently penetrated by linear
crevices, sometimes, though rarely, containing a trace of woody matter.
These calcareous, branching bodies, appear to have been formed by fine
calcareous matter being washed into the casts or cavities, left by the
decay of branches and roots of thickets, buried under drifted sand. The
whole surface of the hill is now undergoing disintegration, and hence
the casts, which are compact and hard, are left projecting. In
calcareous sand at the Cape of Good Hope, I found the casts, described
by Abel, quite similar to these at Bald Head; but their centres are
often filled with black carbonaceous matter not yet removed. It is not
surprising, that the woody matter should have been almost entirely
removed from the casts on Bald Head; for it is certain, that many
centuries must have elapsed since the thickets were buried; at present,
owing to the form and height of the narrow promontory, no sand is
drifted up, and the whole surface, as I have remarked, is wearing away.
We must, therefore, look back to a period when the land stood lower, of
which the French naturalists (See M. Peron “Voyage” tome 1 page 204.)
found evidence in upraised shells of recent species, for the drifting
on Bald Head of the calcareous and quartzose sand, and the consequent
embedment of the vegetable remains. There was only one appearance which
at first made me doubt concerning the origin of the cast,—namely, that
the finer roots from different stems sometimes became united together
into upright plates or veins; but when the manner is borne in mind in
which fine roots often fill up cracks in hard earth, and that these
roots would decay and leave hollows, as well as the stems, there is no
real difficulty in this case. Besides the calcareous branches from the
Cape of Good Hope, I have seen casts, of exactly the same forms, from
Madeira* and from Bermuda; at this latter place, the surrounding
calcareous rocks, judging from the specimens collected by Lieutenant
Nelson, are likewise similar, as is their subaerial formation.
Reflecting on the stratification of the deposit on Bald Head,—on the
irregularly alternating layers of substalagmitic rock,—on the uniformly
sized, and rounded particles, apparently of sea-shells and corals,—on
the abundance of land-shells throughout the mass,—and finally, on the
absolute resemblance of the calcareous casts, to the stumps, roots, and
branches of that kind of vegetation, which would grow on sand-hillocks,
I think there can be no reasonable doubt, notwithstanding the different
opinion of some authors, that a true view of their origin has been here
given.

*(Dr. J. Macaulay has fully described (“Edinb. New Phil. Journ.” volume
29 page 350) the casts from Madeira. He considers (differently from Mr.
Smith of Jordan Hill) these bodies to be corals, and the calcareous
deposit to be of subaqueous origin. His arguments chiefly rest (for his
remarks on their structure are vague) on the great quantity of the
calcareous matter, and on the casts containing animal matter, as shown
by their evolving ammonia. Had Dr. Macaulay seen the enormous masses of
rolled particles of shells and corals on the beach of Ascension, and
especially on coral-reefs; and had he reflected on the effects of
long-continued, gentle winds, in drifting up the finer particles, he
would hardly have advanced the argument of quantity, which is seldom
trustworthy in geology. If the calcareous matter has originated from
disintegrated shells and corals, the presence of animal matter is what
might have been expected. Mr. Anderson analysed for Dr. Macaulay part
of a cast, and he found it composed of:—

Carbonate of lime......73.15 Silica.................11.90 Phosphate of
lime.......8.81 Animal matter...........4.25 Sulphate of lime......a
trace 98.11)

Calcareous deposits, like these of King George’s Sound, are of vast
extent on the Australian shores. Dr. Fitton remarks, that “recent
calcareous breccia (by which term all these deposits are included) was
found during Baudin’s voyage, over a space of no less than twenty-five
degrees of latitude and an equal extent of longitude, on the southern,
western, and north-western coasts.” (For ample details on this
formation consult Dr. Fitton “Appendix to Captain King’s Voyage.” Dr.
Fitton is inclined to attribute a concretionary origin to the branching
bodies: I may remark, that I have seen in beds of sand in La Plata
cylindrical stems which no doubt thus originated; but they differed
much in appearance from these at Bald Head, and the other places above
specified.) It appears also from M. Peron, with whose observations and
opinions on the origin of the calcareous matter and branching casts
mine entirely accord, that the deposit is generally much more
continuous than near King George’s Sound. At Swan River, Archdeacon
Scott states that in one part it extends ten miles inland.
(“Proceedings of the Geolog. Soc.” volume 1 page 320.) Captain Wickham,
moreover, informs me that during his late survey of the western coast,
the bottom of the sea, wherever the vessel anchored, was ascertained,
by crowbars being let down, to consist of white calcareous matter.
Hence it seems that along this coast, as at Bermuda and at Keeling
Atoll, submarine and subaerial deposits are contemporaneously in
process of formation, from the disintegration of marine organic bodies.
The extent of these deposits, considering their origin, is very
striking; and they can be compared in this respect only with the great
coral-reefs of the Indian and Pacific Oceans. In other parts of the
world, for instance in South America, there are SUPERFICIAL calcareous
deposits of great extent, in which not a trace of organic structure is
discoverable; these observations would lead to the inquiry, whether
such deposits may not, also, have been formed from disintegrated shells
and corals.

CAPE OF GOOD HOPE.

After the accounts given by Barrow, Carmichael, Basil Hall, and W.B.
Clarke of the geology of this district, I shall confine myself to a few
observations on the junction of the three principal formations. The
fundamental rock is granite (In several places I observed in the
granite, small dark-coloured balls, composed of minute scales of black
mica in a tough basis. In another place, I found crystals of black
schorl radiating from a common centre. Dr. Andrew Smith found, in the
interior parts of the country, some beautiful specimens of granite,
with silvery mica radiating or rather branching, like moss, from
central points. At the Geological Society, there are specimens of
granite with crystallised feldspar branching and radiating in like
manner.), overlaid by clay-slate: the latter is generally hard, and
glossy from containing minute scales of mica; it alternates with, and
passes into, beds of slightly crystalline, feldspathic, slaty rock.
This clay-slate is remarkable from being in some places (as on the
Lion’s Rump) decomposed, even to the depth of twenty feet, into a
pale-coloured, sandstone-like rock, which has been mistaken, I believe,
by some observers, for a separate formation. I was guided by Dr. Andrew
Smith to a fine junction at Green Point between the granite and
clay-slate: the latter at the distance of a quarter of a mile from the
spot, where the granite appears on the beach (though, probably, the
granite is much nearer underground), becomes slightly more compact and
crystalline. At a less distance, some of the beds of clay-slate are of
a homogeneous texture, and obscurely striped with different zones of
colour, whilst others are obscurely spotted. Within a hundred yards of
the first vein of granite, the clay-slate consists of several
varieties; some compact with a tinge of purple, others glistening with
numerous minute scales of mica and imperfectly crystallised feldspar;
some obscurely granular, others porphyritic with small, elongated spots
of a soft white mineral, which being easily corroded, gives to this
variety a vesicular appearance. Close to the granite, the clay-slate is
changed into a dark-coloured, laminated rock, having a granular
fracture, which is due to imperfect crystals of feldspar, coated by
minute, brilliant scales of mica.

The actual junction between the granitic and clay-slate districts
extends over a width of about two hundred yards, and consists of
irregular masses and of numerous dikes of granite, entangled and
surrounded by the clay- slate: most of the dikes range in a N.W. and
S.E. line, parallel to the cleavage of the slate. As we leave the
junction, thin beds, and lastly, mere films of the altered clay-slate
are seen, quite isolated, as if floating, in the coarsely crystallised
granite; but although completely detached, they all retain traces of
the uniform N.W. and S.E. cleavage. This fact has been observed in
other similar cases, and has been advanced by some eminent geologists
(See M. Keilhau “Theory on Granite” translated in the “Edinburgh New
Philosophical Journal” volume 24 page 402.), as a great difficulty on
the ordinary theory, of granite having been injected whilst liquified;
but if we reflect on the probable state of the lower surface of a
laminated mass, like clay-slate, after having been violently arched by
a body of molten granite, we may conclude that it would be full of
fissures parallel to the planes of cleavage; and that these would be
filled with granite, so that wherever the fissures were close to each
other, mere parting layers or wedges of the slate would depend into the
granite. Should, therefore, the whole body of rock afterwards become
worn down and denuded, the lower ends of these dependent masses or
wedges of slate would be left quite isolated in the granite; yet they
would retain their proper lines of cleavage, from having been united,
whilst the granite was fluid, with a continuous covering of clay-slate.

Following, in company with Dr. A. Smith, the line of junction between
the granite and the slate, as it stretched inland, in a S.E. direction,
we came to a place, where the slate was converted into a fine-grained,
perfectly characterised gneiss, composed of yellow-brown granular
feldspar, of abundant black brilliant mica, and of few and thin laminae
of quartz. From the abundance of the mica in this gneiss, compared with
the small quantity and excessively minute scales, in which it exists in
the glossy clay-slate, we must conclude, that it has been here formed
by the metamorphic action—a circumstance doubted, under nearly similar
circumstances, by some authors. The laminae of the clay-slate are
straight; and it was interesting to observe, that as they assumed the
character of gneiss, they became undulatory with some of the smaller
flexures angular, like the laminae of many true metamorphic schists.

SANDSTONE FORMATION.

This formation makes the most imposing feature in the geology of
Southern Africa. The strata are in many parts horizontal, and attain a
thickness of about two thousand feet. The sandstone varies in
character; it contains little earthy matter, but is often stained with
iron; some of the beds are very fine-grained and quite white; others
are as compact and homogeneous as quartz rock. In some places I
observed a breccia of quartz, with the fragments almost dissolved in a
siliceous paste. Broad veins of quartz, often including large and
perfect crystals, are very numerous; and it is evident in nearly all
the strata, that silica has been deposited from solution in remarkable
quantity. Many of the varieties of quartzite appeared quite like
metamorphic rocks; but from the upper strata being as siliceous as the
lower, and from the undisturbed junctions with the granite, which in
many places can be examined, I can hardly believe that these
sandstone-strata have been exposed to heat. (The Rev. W.B. Clarke,
however, states, to my surprise (“Geolog. Proceedings” volume 3 page
422), that the sandstone in some parts is penetrated by granitic dikes:
such dikes must belong to an epoch altogether subsequent to that when
the molten granite acted on the clay-slate.) On the lines of junction
between these two great formations, I found in several places the
granite decayed to the depth of a few inches, and succeeded, either by
a thin layer of ferruginous shale, or by four or five inches in
thickness of the re-cemented crystals of the granite, on which the
great pile of sandstone immediately rested.

Mr. Schomburgk has described (“Geographical Journal” volume 10 page
246.) a great sandstone formation in Northern Brazil, resting on
granite, and resembling to a remarkable degree, in composition and in
the external form of the land, this formation of the Cape of Good Hope.
The sandstones of the great platforms of Eastern Australia, which also
rest on granite, differ in containing more earthy and less siliceous
matter. No fossil remains have been discovered in these three vast
deposits. Finally, I may add that I did not see any boulders of
far-transported rocks at the Cape of Good Hope, or on the eastern and
western shores of Australia, or at Van Diemen’s Land. In the northern
island of New Zealand, I noticed some large blocks of greenstone, but
whether their parent rock was far distant, I had no opportunity of
determining.



INDEX TO VOLCANIC ISLANDS.


Abel, M., on calcareous casts at the Cape of Good Hope.

Abingdon island.

Abrolhos islands, incrustation on.

Aeriform explosions at Ascension.

Albatross, driven from St. Helena.

Albemarle island.

Albite, at the Galapagos archipelago.

Amygdaloidal cells, half filled.

Amygdaloids, calcareous origin of.

Ascension, arborescent incrustation on rocks of. -absence of dikes,
freedom from volcanic action, and state of lava-streams.

Ascidia, extinction of.

Atlantic Ocean, new volcanic focus in.

Augite, fused.

Australia.

Azores.

Bahia in Brazil, dikes at.

Bailly, M., on the mountains of Mauritius.

Bald Head.

Banks’ Cove.

Barn, The, St. Helena.

Basalt, specific gravity of.

Basaltic coast-mountains at Mauritius. -at St. Helena. -at St. Jago.

Beaumont, M. Elie de, on circular subsidences in lava. -on dikes
indicating elevation. -on inclination of lava-streams. -on laminated
dikes.

Bermuda, calcareous rocks of.

Beudant, M., on bombs. -on jasper. -on laminated trachyte. -on obsidian
of Hungary. -on silex in trachyte.

Bole.

Bombs, volcanic.

Bory St. Vincent, on bombs.

Boulders, absence in Australia and Cape of Good Hope.

Brattle island.

Brewster, Sir D., on a calcareo-animal substance. -on decomposed glass.

Brown, Mr. R., on extinct plants from Van Diemen’s land. -on
sphaerulitic bodies in silicified wood.

Buch, Von, on cavernous lava. -on central volcanoes. -on crystals
sinking in obsidian. -on laminated lava. -on obsidian streams. -on
olivine in basalt. -on superficial calcareous beds in the Canary
islands.

Calcareous deposit at St. Jago affected by heat. -fibrous matter,
entangled in streaks in scoriae. -freestone at Ascension.
-incrustations at Ascension. -sandstone at St. Helena. -superficial
beds at King George’s sound.

Cape of Good Hope.

Carbonic acid, expulsion of, by heat.

Carmichael, Capt., on glassy coatings to dikes.

Casts, calcareous, of branches.

Chalcedonic nodules.

Chalcedony in basalt and in silicified wood.

Chatham island.

Chlorophaeite.

Clarke, Rev. W., on the Cape of Good Hope.

Clay-slate, its decomposition and junction with granite at the Cape of
Good Hope.

Cleavage of clay-slate in Australia.

Cleavage, cross, in sandstone.

Coast denudation at St. Helena.

Columnar basalt.

“Comptes Rendus,” account of volcanic phenomena in the Atlantic.

Concepcion, earthquake of.

Concretions in aqueous and igneous rocks compared. -in tuff. -of
obsidian.

Conglomerate, recent, at St. Jago.

Coquimbo, curious rock of.

Corals, fossil, from Van Diemen’s Land.

Crater, segment of, at the Galapagos. -great central one at St. Helena.
-internal ledges round, and parapet on.

Craters, basaltic, at Ascension. -form of, affected by the trade wind.
-of elevation. -of tuff at Terceira. -of tuff at the Galapagos
archipelago. -their breached state. -small basaltic at St. Jago. —at
the Galapagos archipelago.

Crystallisation favoured by space.

Dartigues, M., on sphaerulites.

Daubeny, Dr., on a basin-formed island. -on fragments in trachyte.

D’Aubuisson on hills of phonolite. -on the composition of obsidian. -on
the lamination of clay-slate.

De la Beche, Sir H., on magnesia in erupted lime. -on specific gravity
of limestones.

Denudation of coast at St. Helena.

Diana’s Peak, St. Helena.

Dieffenbach, Dr., on the Chatham Islands.

Dikes, truncated, on central crateriform ridge of St. Helena. -at St.
Helena; number of; coated by a glossy layer; uniform thickness of.
-great parallel ones at St. Helena. -not observed at Ascension. -of
tuff. -of trap in the plutonic series. -remnants of, extending far into
the sea round St. Helena.

Dislocations at Ascension. -at St. Helena.

Distribution of volcanic islands.

Dolomieu, on decomposed trachyte. -on laminated lava. -on obsidian.

Dree, M., on crystals sinking in lava.

Dufrenoy, M., on the composition of the surface of certain
lava-streams. -on the inclination of tuff-strata.

Eggs of birds embedded at St. Helena. -of turtle at Ascension.

Ejected fragments at Ascension. -at the Galapagos archipelago.

Elevation of St. Helena. -the Galapagos archipelago. -Van Diemen’s
Land, Cape of Good Hope, New Zealand, Australia, and Chatham island.
-of volcanic islands.

Ellis, Rev. W., on ledges within the great crater at Hawaii. -on marine
remains at Otaheite.

Eruption, fissures of.

Extinction of land-shells at St. Helena.

Faraday, Mr., on the expulsion of carbonic acid gas.

Feldspar, fusibility of. -in radiating crystals. -Labrador, ejected.

Feldspathic lavas. -at St. Helena. -rock, alternating with obsidian.
-lamination, and origin of.

Fernando Noronha.

Ferruginous superficial beds.

Fibrous calcareous matter at St. Jago.

Fissures of eruption.

Fitton, Dr., on calcareous breccia.

Flagstaff Hill, St. Helena.

Fleurian de Bellevue on sphaerulites.

Fluidity of lavas.

Forbes, Professor, on the structure of glaciers.

Fragments ejected at Ascension. -at the Galapagos archipelago.

Freshwater Bay.

Fuerteventura (Feurteventura), calcareous beds of.

Galapagos archipelago. -parapets round craters.

Gay Lussac, on the expulsion of carbonic acid gas.

Glaciers, their structure.

Glossiness of texture, origin of.

Gneiss, derived from clay-slate. -with a great embedded fragment.

Gneiss-granite, form of hills of.

Good Hope, Cape of.

Gorges, narrow, at St. Helena.

Granite, junction with clay-slate, at the Cape of Good Hope.

Granitic ejected fragments.

Gravity, specific, of lavas.

Gypsum, at Ascension. -in volcanic strata at St. Helena. -on surface of
the ground at ditto.

Hall, Sir J., on the expulsion of carbonic acid gas.

Heat, action of, on calcareous matter.

Hennah, Mr., on ashes at Ascension.

Henslow, Prof., on chalcedony.

Hoffmann, on decomposed trachyte.

Holland, Dr., on Iceland.

Horner, Mr., on a calcareo-animal substance. -on fusibility of
feldspar.

Hubbard, Dr., on dikes.

Humboldt on ejected fragments. -on obsidian formations. -on parapets
round craters. -on sphaerulites.

Hutton on amygdaloids.

Hyalite in decomposed trachyte.

Iceland, stratification of the circumferential hills.

Islands, volcanic, distribution of. -their elevation.

Incrustation, on St. Paul’s rocks.

Incrustations, calcareous, at Ascension.

Jago, St.

James island.

Jasper, origin of.

Jonnes, M. Moreau de, on craters affected by wind.

Juan Fernandez.

Keilhau, M., on granite.

Kicker Rock.

King George’s sound.

Labrador feldspar, ejected.

Lakes at bases of volcanoes.

Lamination of volcanic rocks.

Land-shells, extinct, at St. Helena.

Lanzarote, calcareous beds of.

Lava, adhesion to sides of a gorge. -feldspathic. -with cells
semi-amygdaloidal.

Lavas, specific gravity of.

Lava-streams blending together at St. Jago. -composition of surface of.
-differences in the state of their surfaces. -extreme thinness of.
-heaved up into hillocks at the Galapagos archipelago. -their fluidity.
-with irregular hummocks at Ascension.

Lead, separation from silver.

Lesson, M., on craters at Ascension.

Leucite.

Lime, sulphate of, at Ascension.

Lonsdale, Mr., on fossil-corals from Van Diemen’s land.

Lot, St. Helena.

Lyell, Mr., on craters of elevation. -on embedded turtles’ eggs. -on
glossy coating to dikes.

Macaulay, Dr., on calcareous casts at Madeira.

MacCulloch, Dr., on an amygdaloid. -on chlorophaeite. -on laminated
pitchstone.

Mackenzie, Sir G., on cavernous lava-streams. -on glossy coatings to
dikes. -on obsidian streams. -on stratification in Iceland.

Madeira, calcareous casts at.

“Magazine, Nautical,” account of volcanic phenomena in the Atlantic.

Marekanite.

Mauritius, crater of elevation of.

Mica, in rounded nodules. -origin in metamorphic slate. -radiating form
of.

Miller, Prof., on ejected Labrador feldspar. -on quartz crystals in
obsidian beds.

Mitchell, Sir T., on bombs. -on the Australian valleys.

Mud streams at the Galapagos archipelago.

Narborough island.

Nelson, Lieut., on the Bermuda islands.

New Caledonia.

New Red sandstone, cross cleavage of.

New South Wales.

New Zealand.

Nulliporae (fossil), resembling concretions.

Obsidian, absent at the Galapagos archipelago. -bombs of. -composition
and origin of. -crystals of feldspar sink in. -its irruption from lofty
craters. -passage of beds into. -specific gravity of. -streams of.

Olivine decomposed at St. Jago. -at Van Diemen’s land. -in the lavas at
the Galapagos archipelago.

Oolitic structure of recent calcareous beds at St. Helena.

Otaheite.

Oysters, extinction of.

Panza islands, laminated trachyte of.

Pattinson, Mr., on the separation of lead and silver.

Paul’s, St., rocks of.

Pearlstone.

Peperino.

Peron, M., on calcareous rocks of Australia.

Phonolite, hills of. -laminated. -with more fusible hornblende.

Pitchstone. -dikes of.

Plants, extinct.

Plutonic rocks, separation of constituent parts of, by gravity.

Porto Praya.

Prevost, M. C., on rarity of great dislocations in volcanic islands.

Prosperous hill, St. Helena.

Pumice, absent at the Galapagos archipelago. -laminated.

Puy de Dome, trachyte of.

Quail island, St. Jago.

Quartz, crystals of, in beds alternating with obsidian. -crystallised
in sandstone. -fusibility of. -rock, mottled from metamorphic action
with earthy matter.

Red hill.

Resin-like altered scoriae.

Rio de Janeiro, gneiss of.

Robert, M., on strata of Iceland.

Rogers, Professor, on curved lines of elevation.

Salses, compared with tuff craters.

Salt deposited by the sea. -in volcanic strata. -lakes of, in craters.

Sandstone of Brazil. -of the Cape of Good Hope. -platforms of, in New
South Wales.

Schorl, radiating.

Scrope, Mr. P., on laminated trachyte. -on obsidian. -on separation of
trachyte and basalt. -on silex in trachyte. -on sphaerulites.

Seale, Mr., geognosy of St. Helena. -on dikes. -on embedded birds’
bones.

Seale, on extinct shells of St. Helena.

Sedgwick, Professor, on concretions.

Septaria, in concretions in tuff.

Serpulae on upraised rocks.

Seychelles.

Shells, colour of, affected by light. -from Van Diemen’s land. -land,
extinct, at St. Helena. -particles of, drifted by the wind at St.
Helena.

Shelly matter deposited by the waves.

Siau, M., on ripples.

Signal Post Hill.

Silica, deposited by steam. -large proportion of, in obsidian.
-specific gravity of.

Siliceous sinter.

Smith, Dr. A., on junction of granite and clay-slate.

Spallanzani on decomposed trachyte.

Specific gravity of recent calcareous rocks and of limestone. -of
lavas.

Sphaerulites in glass and in silicified wood. -in obsidian.

Sowerby, Mr. G.B., on fossil-shells from Van Diemen’s land. -from St.
Jago. -land-shells from St. Helena.

St. Helena. -crater of elevation of.

St. Jago, crater of elevation of. -effects of calcareous matter on
lava.

St. Paul’s rocks.

Stokes, Mr., collections of sphaerulites and of obsidians.

Stony-top, Little. -Great.

Stratification of sandstone in New South Wales.

Streams of obsidian.

Stutchbury, Mr., on marine remains at Otaheite.

Subsided space at Ascension.

Tahiti.

Talus, stratified, within tuff craters.

Terceira.

Tertiary deposit of St. Jago.

Trachyte, absent at the Galapagos archipelago. -at Ascension. -at
Terceira. -decomposition of, by steam. -its lamination. -its separation
from basalt. -softened at Ascension. -specific gravity of. -with
singular veins.

Trap-dikes in the plutonic series. -at King George’s sound.

Travertin at Van Diemen’s land.

Tropic-bird, now rare, at St. Helena.

Tuff, craters of. -their breached state. -peculiar kind of.

Turner, Mr., on the separation of molten metals.

Tyerman and Bennett on marine remains at Huaheine.

Valleys, gorge-like, at St. Helena. -in New South Wales. -in St. Jago.

Van Diemen’s land.

Veins in trachyte. -of jasper.

Vincent, Bory St., on bombs.

Volcanic bombs. -island in process of formation in the Atlantic.
-islands, their distribution.

Wacke, its passage into lava.

Wackes, argillaceous.

Webster, Dr., on a basin-formed island. -on gypsum at Ascension.

White, Martin, on soundings.

Wind, effects of, on the form of craters.






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